auto merge of #2038 : SimonSapin/servo/min-height-collapse, r=pcwalton
auto merge of #2038 : SimonSapin/servo/min-height-collapse, r=pcwalton

[Relevant spec](http://dev.w3.org/csswg/css2/box.html#collapsing-margins):

> top and bottom margins of a box that does not establish a new block formatting context and that has zero computed 'min-height', zero or 'auto' computed 'height', and no in-flow children 

bjwbell/servo,runarberg/servo,pyfisch/servo,upsuper/servo,szeged/servo,hyowon/servo,tempbottle/servo,walac/servo,KiChjang/servo,walac/servo,rentongzhang/servo,upsuper/servo,zentner-kyle/servo,froydnj/servo,nnethercote/servo,Adenilson/prototype-viewing-distance,rixrix/servo,GreenRecycleBin/servo,SimonSapin/servo,larsbergstrom/servo,wpgallih/servo,DominoTree/servo,A-deLuna/servo,emilio/servo,wpgallih/servo,evilpie/servo,peterjoel/servo,mdibaiee/servo,evilpie/servo,zentner-kyle/servo,DominoTree/servo,tschneidereit/servo,DominoTree/servo,canaltinova/servo,nick-thompson/servo,mt2d2/servo,wpgallih/servo,jimberlage/servo,samfoo/servo,saneyuki/servo,peterjoel/servo,ryancanhelpyou/servo,rnestler/servo,nrc/servo,eddyb/servo,dmarcos/servo,aweinstock314/servo,snf/servo,Adenilson/prototype-viewing-distance,peterjoel/servo,shrenikgala/servo,walac/servo,chotchki/servo,s142857/servo,deokjinkim/servo,rentongzhang/servo,anthgur/servo,aidanhs/servo,chotchki/servo,nnethercote/servo,akosel/servo,srbhklkrn/SERVOENGINE,rixrix/servo,notriddle/servo,runarberg/servo,mbrubeck/servo,KiChjang/servo,cbrewster/servo,karlito40/servo,tempbottle/servo,samfoo/servo,fiji-flo/servo,evilpie/servo,juzer10/servo,AnthonyBroadCrawford/servo,ConnorGBrewster/servo,mt2d2/servo,g-k/servo,dsandeephegde/servo,snf/servo,aidanhs/servo,zhangjunlei26/servo,larsbergstrom/servo,saneyuki/servo,eddyb/servo,tafia/servo,jimberlage/servo,bfrohs/servo,j3parker/servo,samfoo/servo,pgonda/servo,pyecs/servo,ryancanhelpyou/servo,thiagopnts/servo,evilpie/servo,CJ8664/servo,nnethercote/servo,SimonSapin/servo,nerith/servo,shrenikgala/servo,rentongzhang/servo,RenaudParis/servo,dhananjay92/servo,peterjoel/servo,sadmansk/servo,tafia/servo,avadacatavra/servo,A-deLuna/servo,avadacatavra/servo,KiChjang/servo,emilio/servo,mattnenterprise/servo,codemac/servo,g-k/servo,anthgur/servo,snf/servo,michaelwu/servo,szeged/servo,jlegendary/servo,dagnir/servo,upsuper/servo,meh/servo,brendandahl/servo,youprofit/servo,hyowon/servo,wpgallih/servo,GreenRecycleBin/servo,michaelwu/servo,jgraham/servo,caldwell/servo,splav/servo,sadmansk/servo,s142857/servo,canaltinova/servo,j3parker/servo,sadmansk/servo,AnthonyBroadCrawford/servo,cbrewster/servo,pgonda/servo,mukilan/servo,ruud-v-a/servo,cbrewster/servo,karlito40/servo,rixrix/servo,jdramani/servo,froydnj/servo,dagnir/servo,saneyuki/servo,kindersung/servo,emilio/servo,srbhklkrn/SERVOENGINE,GreenRecycleBin/servo,dvberkel/servo,saneyuki/servo,seanmonstar/servo,michaelwu/servo,rixrix/servo,rnestler/servo,ConnorGBrewster/servo,rentongzhang/servo,meh/servo,dmarcos/servo,CJ8664/servo,dati91/servo,indykish/servo,notriddle/servo,codemac/servo,mattnenterprise/servo,aweinstock314/servo,wpgallih/servo,emilio/servo,nick-thompson/servo,A-deLuna/servo,brendandahl/servo,peterjoel/servo,evilpie/servo,jimberlage/servo,snf/servo,fiji-flo/servo,larsbergstrom/servo,youprofit/servo,evilpie/servo,juzer10/servo,huonw/servo,dati91/servo,echochamber/servo,saratang/servo,SimonSapin/servo,meh/servo,dagnir/servo,kindersung/servo,walac/servo,mattnenterprise/servo,pyfisch/servo,aweinstock314/servo,zhangjunlei26/servo,dsandeephegde/servo,saratang/servo,snf/servo,huonw/servo,RenaudParis/servo,pgonda/servo,g-k/servo,mattnenterprise/servo,youprofit/servo,GyrosOfWar/servo,sadmansk/servo,echochamber/servo,froydnj/servo,jdramani/servo,huonw/servo,meh/servo,dati91/servo,wartman4404/servo,tafia/servo,codemac/servo,echochamber/servo,avadacatavra/servo,DominoTree/servo,GyrosOfWar/servo,Adenilson/prototype-viewing-distance,CJ8664/servo,ConnorGBrewster/servo,jdramani/servo,tafia/servo,ryancanhelpyou/servo,thiagopnts/servo,paulrouget/servo,ruud-v-a/servo,aweinstock314/servo,SimonSapin/servo,huonw/servo,DominoTree/servo,akosel/servo,mbrubeck/servo,zentner-kyle/servo,rnestler/servo,froydnj/servo,pgonda/servo,brendandahl/servo,akosel/servo,CJ8664/servo,runarberg/servo,larsbergstrom/servo,SimonSapin/servo,rixrix/servo,bjwbell/servo,saneyuki/servo,brendandahl/servo,jlegendary/servo,vks/servo,jdramani/servo,tafia/servo,juzer10/servo,zhangjunlei26/servo,saneyuki/servo,nerith/servo,ryancanhelpyou/servo,huonw/servo,ConnorGBrewster/servo,shrenikgala/servo,notriddle/servo,canaltinova/servo,notriddle/servo,g-k/servo,AnthonyBroadCrawford/servo,Shraddha512/servo,zhangjunlei26/servo,GreenRecycleBin/servo,cbrewster/servo,caldwell/servo,avadacatavra/servo,akosel/servo,thiagopnts/servo,paulrouget/servo,KiChjang/servo,srbhklkrn/SERVOENGINE,jgraham/servo,larsbergstrom/servo,pyfisch/servo,indykish/servo,Adenilson/prototype-viewing-distance,meh/servo,jdramani/servo,mt2d2/servo,aidanhs/servo,tempbottle/servo,karlito40/servo,zhangjunlei26/servo,emilio/servo,bfrohs/servo,youprofit/servo,zentner-kyle/servo,dsandeephegde/servo,srbhklkrn/SERVOENGINE,A-deLuna/servo,brendandahl/servo,samfoo/servo,mukilan/servo,wartman4404/servo,anthgur/servo,michaelwu/servo,avadacatavra/servo,saratang/servo,szeged/servo,avadacatavra/servo,juzer10/servo,pyfisch/servo,upsuper/servo,bjwbell/servo,dati91/servo,deokjinkim/servo,dmarcos/servo,DominoTree/servo,paulrouget/servo,saneyuki/servo,aweinstock314/servo,fiji-flo/servo,pyfisch/servo,indykish/servo,nrc/servo,peterjoel/servo,vks/servo,GreenRecycleBin/servo,saratang/servo,dhananjay92/servo,mbrubeck/servo,Adenilson/prototype-viewing-distance,seanmonstar/servo,cbrewster/servo,nnethercote/servo,deokjinkim/servo,saneyuki/servo,echochamber/servo,hyowon/servo,mdibaiee/servo,tschneidereit/servo,walac/servo,s142857/servo,deokjinkim/servo,nerith/servo,dsandeephegde/servo,hyowon/servo,RenaudParis/servo,boghison/servo,samfoo/servo,samfoo/servo,nrc/servo,dhananjay92/servo,steveklabnik/servo,luniv/servo,ryancanhelpyou/servo,CJ8664/servo,meh/servo,dati91/servo,Adenilson/prototype-viewing-distance,Shraddha512/servo,fiji-flo/servo,chotchki/servo,GyrosOfWar/servo,saratang/servo,tempbottle/servo,KiChjang/servo,eddyb/servo,emilio/servo,fiji-flo/servo,anthgur/servo,rnestler/servo,pyecs/servo,thiagopnts/servo,michaelwu/servo,WriterOfAlicrow/servo,upsuper/servo,eddyb/servo,ryancanhelpyou/servo,akosel/servo,szeged/servo,saneyuki/servo,pyfisch/servo,nnethercote/servo,zhangjunlei26/servo,akosel/servo,deokjinkim/servo,nrc/servo,echochamber/servo,canaltinova/servo,sadmansk/servo,saneyuki/servo,mdibaiee/servo,j3parker/servo,GyrosOfWar/servo,dsandeephegde/servo,splav/servo,dati91/servo,chotchki/servo,nick-thompson/servo,zentner-kyle/servo,jdramani/servo,nrc/servo,nick-thompson/servo,evilpie/servo,nick-thompson/servo,wartman4404/servo,dvberkel/servo,mdibaiee/servo,rentongzhang/servo,steveklabnik/servo,notriddle/servo,luniv/servo,codemac/servo,indykish/servo,nrc/servo,jimberlage/servo,boghison/servo,rnestler/servo,GreenRecycleBin/servo,WriterOfAlicrow/servo,thiagopnts/servo,KiChjang/servo,tschneidereit/servo,codemac/servo,juzer10/servo,indykish/servo,KiChjang/servo,caldwell/servo,larsbergstrom/servo,dhananjay92/servo,steveklabnik/servo,peterjoel/servo,bjwbell/servo,aweinstock314/servo,echochamber/servo,larsbergstrom/servo,huonw/servo,luniv/servo,DominoTree/servo,srbhklkrn/SERVOENGINE,wpgallih/servo,tempbottle/servo,s142857/servo,akosel/servo,wartman4404/servo,nerith/servo,szeged/servo,steveklabnik/servo,seanmonstar/servo,dvberkel/servo,tempbottle/servo,srbhklkrn/SERVOENGINE,chotchki/servo,rentongzhang/servo,mukilan/servo,seanmonstar/servo,canaltinova/servo,RenaudParis/servo,juzer10/servo,GreenRecycleBin/servo,jimberlage/servo,ruud-v-a/servo,zentner-kyle/servo,j3parker/servo,RenaudParis/servo,j3parker/servo,pyecs/servo,anthgur/servo,walac/servo,boghison/servo,jdramani/servo,ConnorGBrewster/servo,aweinstock314/servo,canaltinova/servo,bjwbell/servo,karlito40/servo,SimonSapin/servo,mdibaiee/servo,DominoTree/servo,codemac/servo,Shraddha512/servo,steveklabnik/servo,jgraham/servo,dagnir/servo,SimonSapin/servo,emilio/servo,tschneidereit/servo,rixrix/servo,peterjoel/servo,sadmansk/servo,pyecs/servo,GyrosOfWar/servo,KiChjang/servo,paulrouget/servo,bfrohs/servo,zentner-kyle/servo,KiChjang/servo,emilio/servo,dvberkel/servo,ryancanhelpyou/servo,brendandahl/servo,aidanhs/servo,notriddle/servo,shrenikgala/servo,jimberlage/servo,aidanhs/servo,youprofit/servo,peterjoel/servo,thiagopnts/servo,WriterOfAlicrow/servo,caldwell/servo,runarberg/servo,DominoTree/servo,fiji-flo/servo,nnethercote/servo,larsbergstrom/servo,paulrouget/servo,boghison/servo,larsbergstrom/servo,RenaudParis/servo,nnethercote/servo,mukilan/servo,pyfisch/servo,brendandahl/servo,kindersung/servo,karlito40/servo,mt2d2/servo,Shraddha512/servo,avadacatavra/servo,indykish/servo,upsuper/servo,A-deLuna/servo,mbrubeck/servo,bjwbell/servo,notriddle/servo,rixrix/servo,larsbergstrom/servo,seanmonstar/servo,fiji-flo/servo,pyecs/servo,eddyb/servo,echochamber/servo,cbrewster/servo,huonw/servo,indykish/servo,anthgur/servo,youprofit/servo,GreenRecycleBin/servo,caldwell/servo,dsandeephegde/servo,GreenRecycleBin/servo,luniv/servo,wartman4404/servo,j3parker/servo,pyecs/servo,Shraddha512/servo,GyrosOfWar/servo,s142857/servo,pyfisch/servo,steveklabnik/servo,eddyb/servo,runarberg/servo,WriterOfAlicrow/servo,splav/servo,avadacatavra/servo,jlegendary/servo,codemac/servo,shrenikgala/servo,jimberlage/servo,dagnir/servo,samfoo/servo,deokjinkim/servo,nnethercote/servo,thiagopnts/servo,splav/servo,bjwbell/servo,g-k/servo,nerith/servo,A-deLuna/servo,dhananjay92/servo,AnthonyBroadCrawford/servo,szeged/servo,ConnorGBrewster/servo,paulrouget/servo,evilpie/servo,tschneidereit/servo,kindersung/servo,splav/servo,aidanhs/servo,Shraddha512/servo,g-k/servo,vks/servo,nnethercote/servo,steveklabnik/servo,jlegendary/servo,rixrix/servo,ConnorGBrewster/servo,pgonda/servo,cbrewster/servo,thiagopnts/servo,dvberkel/servo,splav/servo,hyowon/servo,froydnj/servo,vks/servo,pgonda/servo,juzer10/servo,mattnenterprise/servo,tafia/servo,jimberlage/servo,tschneidereit/servo,splav/servo,caldwell/servo,nick-thompson/servo,runarberg/servo,mbrubeck/servo,mdibaiee/servo,caldwell/servo,wpgallih/servo,kindersung/servo,emilio/servo,shrenikgala/servo,zhangjunlei26/servo,zhangjunlei26/servo,j3parker/servo,dhananjay92/servo,AnthonyBroadCrawford/servo,s142857/servo,anthgur/servo,g-k/servo,tafia/servo,dagnir/servo,runarberg/servo,vks/servo,canaltinova/servo,paulrouget/servo,ruud-v-a/servo,pgonda/servo,nrc/servo,jlegendary/servo,WriterOfAlicrow/servo,dagnir/servo,mt2d2/servo,boghison/servo,wartman4404/servo,dmarcos/servo,vks/servo,mattnenterprise/servo,Adenilson/prototype-viewing-distance,cbrewster/servo,paulrouget/servo,mukilan/servo,aidanhs/servo,GyrosOfWar/servo,hyowon/servo,nick-thompson/servo,eddyb/servo,WriterOfAlicrow/servo,sadmansk/servo,jgraham/servo,saratang/servo,splav/servo,nnethercote/servo,sadmansk/servo,mukilan/servo,wartman4404/servo,rnestler/servo,mdibaiee/servo,evilpie/servo,nerith/servo,froydnj/servo,mattnenterprise/servo,dsandeephegde/servo,peterjoel/servo,karlito40/servo,vks/servo,pyecs/servo,paulrouget/servo,dhananjay92/servo,youprofit/servo,pyfisch/servo,notriddle/servo,indykish/servo,mbrubeck/servo,fiji-flo/servo,szeged/servo,ruud-v-a/servo,bfrohs/servo,snf/servo,dmarcos/servo,luniv/servo,karlito40/servo,canaltinova/servo,indykish/servo,jgraham/servo,hyowon/servo,A-deLuna/servo,pyfisch/servo,chotchki/servo,szeged/servo,ConnorGBrewster/servo,dati91/servo,KiChjang/servo,eddyb/servo,dvberkel/servo,CJ8664/servo,AnthonyBroadCrawford/servo,wpgallih/servo,dsandeephegde/servo,DominoTree/servo,mattnenterprise/servo,szeged/servo,dmarcos/servo,srbhklkrn/SERVOENGINE,splav/servo,mbrubeck/servo,mbrubeck/servo,meh/servo,upsuper/servo,boghison/servo,notriddle/servo,anthgur/servo,Shraddha512/servo,saratang/servo,RenaudParis/servo,jimberlage/servo,dvberkel/servo,jgraham/servo,paulrouget/servo,dmarcos/servo,szeged/servo,luniv/servo,kindersung/servo,walac/servo,rentongzhang/servo,mt2d2/servo,CJ8664/servo,splav/servo,shrenikgala/servo,zhangjunlei26/servo,notriddle/servo,upsuper/servo,seanmonstar/servo,nerith/servo,rnestler/servo,boghison/servo,rnestler/servo,jgraham/servo,mt2d2/servo,s142857/servo,bfrohs/servo,jlegendary/servo,SimonSapin/servo,WriterOfAlicrow/servo,CJ8664/servo,jlegendary/servo,kindersung/servo,deokjinkim/servo,dati91/servo,bfrohs/servo,ruud-v-a/servo,tempbottle/servo,luniv/servo,rixrix/servo,AnthonyBroadCrawford/servo,wpgallih/servo,tschneidereit/servo,snf/servo,michaelwu/servo,emilio/servo