auto merge of #2130 : mbrubeck/servo/frametree-copy-fix, r=larsbergstrom
auto merge of #2130 : mbrubeck/servo/frametree-copy-fix, r=larsbergstrom

We are calling `get` when we should call `borrow` or `borrow_mut`.  This is causing bugs with iframes because we append the child frame to a copy of the parent's `children` vector rather than the original.

r? @larsbergstrom

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