auto merge of #3823 : glennw/servo/default-opts, r=mbrubeck
auto merge of #3823 : glennw/servo/default-opts, r=mbrubeck

This is required for unit tests like the image cache task, which can pass through code paths that query the command line options.

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