compositing: Use the pre-existing script channel in the same-domain iframe case instead of creating a new one.
compositing: Use the pre-existing script channel in the same-domain
iframe case instead of creating a new one.

Fixes test timeouts.

aweinstock314/servo,pgonda/servo,larsbergstrom/servo,rentongzhang/servo,bjwbell/servo,akosel/servo,RenaudParis/servo,ConnorGBrewster/servo,szeged/servo,kindersung/servo,bjwbell/servo,nick-thompson/servo,aidanhs/servo,wartman4404/servo,mbrubeck/servo,vks/servo,rnestler/servo,WriterOfAlicrow/servo,SimonSapin/servo,saratang/servo,nnethercote/servo,canaltinova/servo,deokjinkim/servo,rnestler/servo,RenaudParis/servo,WriterOfAlicrow/servo,emilio/servo,paulrouget/servo,dvberkel/servo,caldwell/servo,anthgur/servo,nick-thompson/servo,s142857/servo,huonw/servo,huonw/servo,jdramani/servo,splav/servo,RenaudParis/servo,rnestler/servo,tafia/servo,g-k/servo,j3parker/servo,upsuper/servo,nrc/servo,jlegendary/servo,peterjoel/servo,emilio/servo,pyecs/servo,DominoTree/servo,brendandahl/servo,pyfisch/servo,mdibaiee/servo,pyecs/servo,tempbottle/servo,dvberkel/servo,g-k/servo,nick-thompson/servo,akosel/servo,bjwbell/servo,youprofit/servo,eddyb/servo,deokjinkim/servo,meh/servo,RenaudParis/servo,pyfisch/servo,boghison/servo,codemac/servo,mbrubeck/servo,sadmansk/servo,pyfisch/servo,avadacatavra/servo,emilio/servo,KiChjang/servo,pyfisch/servo,zentner-kyle/servo,emilio/servo,CJ8664/servo,pyfisch/servo,AnthonyBroadCrawford/servo,deokjinkim/servo,pgonda/servo,avadacatavra/servo,caldwell/servo,cbrewster/servo,deokjinkim/servo,mattnenterprise/servo,WriterOfAlicrow/servo,zentner-kyle/servo,dvberkel/servo,splav/servo,SimonSapin/servo,aidanhs/servo,larsbergstrom/servo,tschneidereit/servo,rnestler/servo,g-k/servo,aidanhs/servo,thiagopnts/servo,runarberg/servo,mbrubeck/servo,tafia/servo,pyecs/servo,meh/servo,tafia/servo,cbrewster/servo,paulrouget/servo,DominoTree/servo,youprofit/servo,eddyb/servo,nerith/servo,j3parker/servo,AnthonyBroadCrawford/servo,rentongzhang/servo,larsbergstrom/servo,cbrewster/servo,tschneidereit/servo,SimonSapin/servo,walac/servo,szeged/servo,thiagopnts/servo,mbrubeck/servo,A-deLuna/servo,KiChjang/servo,codemac/servo,canaltinova/servo,A-deLuna/servo,bjwbell/servo,wartman4404/servo,s142857/servo,vks/servo,j3parker/servo,dhananjay92/servo,tschneidereit/servo,nnethercote/servo,deokjinkim/servo,DominoTree/servo,larsbergstrom/servo,dati91/servo,notriddle/servo,wartman4404/servo,bjwbell/servo,caldwell/servo,larsbergstrom/servo,A-deLuna/servo,splav/servo,splav/servo,steveklabnik/servo,youprofit/servo,steveklabnik/servo,rnestler/servo,aidanhs/servo,s142857/servo,emilio/servo,ryancanhelpyou/servo,CJ8664/servo,KiChjang/servo,AnthonyBroadCrawford/servo,notriddle/servo,thiagopnts/servo,pgonda/servo,cbrewster/servo,fiji-flo/servo,walac/servo,aweinstock314/servo,pyfisch/servo,CJ8664/servo,mdibaiee/servo,anthgur/servo,dsandeephegde/servo,szeged/servo,meh/servo,zentner-kyle/servo,nnethercote/servo,youprofit/servo,CJ8664/servo,dsandeephegde/servo,sadmansk/servo,paulrouget/servo,codemac/servo,jdramani/servo,dhananjay92/servo,runarberg/servo,akosel/servo,A-deLuna/servo,pgonda/servo,wartman4404/servo,anthgur/servo,peterjoel/servo,walac/servo,AnthonyBroadCrawford/servo,akosel/servo,tempbottle/servo,nick-thompson/servo,thiagopnts/servo,KiChjang/servo,thiagopnts/servo,dsandeephegde/servo,mattnenterprise/servo,caldwell/servo,avadacatavra/servo,dvberkel/servo,saneyuki/servo,akosel/servo,upsuper/servo,peterjoel/servo,SimonSapin/servo,caldwell/servo,A-deLuna/servo,jimberlage/servo,aweinstock314/servo,aidanhs/servo,ConnorGBrewster/servo,jlegendary/servo,boghison/servo,jimberlage/servo,WriterOfAlicrow/servo,mattnenterprise/servo,upsuper/servo,sadmansk/servo,CJ8664/servo,eddyb/servo,meh/servo,brendandahl/servo,szeged/servo,boghison/servo,AnthonyBroadCrawford/servo,dati91/servo,mbrubeck/servo,saneyuki/servo,nnethercote/servo,steveklabnik/servo,notriddle/servo,aweinstock314/servo,larsbergstrom/servo,notriddle/servo,canaltinova/servo,WriterOfAlicrow/servo,mattnenterprise/servo,notriddle/servo,meh/servo,nrc/servo,notriddle/servo,nrc/servo,ryancanhelpyou/servo,splav/servo,splav/servo,walac/servo,ConnorGBrewster/servo,pyfisch/servo,upsuper/servo,nrc/servo,boghison/servo,peterjoel/servo,aidanhs/servo,pgonda/servo,walac/servo,jlegendary/servo,canaltinova/servo,walac/servo,saratang/servo,brendandahl/servo,j3parker/servo,tafia/servo,mbrubeck/servo,notriddle/servo,aweinstock314/servo,tempbottle/servo,dsandeephegde/servo,huonw/servo,vks/servo,huonw/servo,DominoTree/servo,paulrouget/servo,runarberg/servo,paulrouget/servo,rentongzhang/servo,peterjoel/servo,rnestler/servo,KiChjang/servo,pgonda/servo,dagnir/servo,paulrouget/servo,rentongzhang/servo,A-deLuna/servo,avadacatavra/servo,mdibaiee/servo,jlegendary/servo,steveklabnik/servo,tschneidereit/servo,zentner-kyle/servo,saneyuki/servo,AnthonyBroadCrawford/servo,splav/servo,tschneidereit/servo,peterjoel/servo,nick-thompson/servo,DominoTree/servo,bjwbell/servo,splav/servo,nnethercote/servo,anthgur/servo,jlegendary/servo,g-k/servo,youprofit/servo,WriterOfAlicrow/servo,KiChjang/servo,anthgur/servo,caldwell/servo,s142857/servo,jimberlage/servo,nerith/servo,KiChjang/servo,GyrosOfWar/servo,tschneidereit/servo,CJ8664/servo,rnestler/servo,codemac/servo,WriterOfAlicrow/servo,SimonSapin/servo,tempbottle/servo,larsbergstrom/servo,jdramani/servo,brendandahl/servo,zentner-kyle/servo,ConnorGBrewster/servo,nnethercote/servo,mattnenterprise/servo,emilio/servo,dati91/servo,pyfisch/servo,GyrosOfWar/servo,paulrouget/servo,aweinstock314/servo,peterjoel/servo,tafia/servo,g-k/servo,tempbottle/servo,bjwbell/servo,dagnir/servo,SimonSapin/servo,nnethercote/servo,runarberg/servo,mdibaiee/servo,jdramani/servo,upsuper/servo,pgonda/servo,jimberlage/servo,nrc/servo,emilio/servo,kindersung/servo,DominoTree/servo,nick-thompson/servo,nick-thompson/servo,saratang/servo,AnthonyBroadCrawford/servo,ryancanhelpyou/servo,DominoTree/servo,dati91/servo,pyfisch/servo,CJ8664/servo,avadacatavra/servo,ryancanhelpyou/servo,cbrewster/servo,fiji-flo/servo,zentner-kyle/servo,eddyb/servo,nerith/servo,codemac/servo,dvberkel/servo,RenaudParis/servo,mattnenterprise/servo,avadacatavra/servo,emilio/servo,meh/servo,codemac/servo,nrc/servo,splav/servo,jimberlage/servo,nnethercote/servo,splav/servo,larsbergstrom/servo,notriddle/servo,pyecs/servo,anthgur/servo,brendandahl/servo,pyfisch/servo,anthgur/servo,fiji-flo/servo,nnethercote/servo,cbrewster/servo,notriddle/servo,thiagopnts/servo,eddyb/servo,kindersung/servo,j3parker/servo,vks/servo,ConnorGBrewster/servo,vks/servo,nnethercote/servo,nerith/servo,vks/servo,mbrubeck/servo,fiji-flo/servo,mdibaiee/servo,cbrewster/servo,DominoTree/servo,pyecs/servo,dagnir/servo,canaltinova/servo,szeged/servo,nerith/servo,youprofit/servo,canaltinova/servo,fiji-flo/servo,j3parker/servo,mattnenterprise/servo,larsbergstrom/servo,dvberkel/servo,CJ8664/servo,dsandeephegde/servo,brendandahl/servo,dagnir/servo,huonw/servo,huonw/servo,ryancanhelpyou/servo,dagnir/servo,boghison/servo,upsuper/servo,saneyuki/servo,saneyuki/servo,avadacatavra/servo,kindersung/servo,saratang/servo,kindersung/servo,paulrouget/servo,rnestler/servo,RenaudParis/servo,szeged/servo,dvberkel/servo,aweinstock314/servo,steveklabnik/servo,sadmansk/servo,saneyuki/servo,pyecs/servo,avadacatavra/servo,eddyb/servo,walac/servo,akosel/servo,rentongzhang/servo,tempbottle/servo,jlegendary/servo,sadmansk/servo,sadmansk/servo,jimberlage/servo,steveklabnik/servo,dsandeephegde/servo,peterjoel/servo,thiagopnts/servo,GyrosOfWar/servo,s142857/servo,GyrosOfWar/servo,jimberlage/servo,youprofit/servo,thiagopnts/servo,meh/servo,szeged/servo,larsbergstrom/servo,emilio/servo,tafia/servo,ConnorGBrewster/servo,upsuper/servo,saneyuki/servo,tschneidereit/servo,jimberlage/servo,steveklabnik/servo,saneyuki/servo,szeged/servo,dhananjay92/servo,nerith/servo,GyrosOfWar/servo,saneyuki/servo,ryancanhelpyou/servo,KiChjang/servo,ryancanhelpyou/servo,fiji-flo/servo,wartman4404/servo,jdramani/servo,nerith/servo,rentongzhang/servo,dsandeephegde/servo,KiChjang/servo,dagnir/servo,canaltinova/servo,SimonSapin/servo,dagnir/servo,canaltinova/servo,DominoTree/servo,caldwell/servo,SimonSapin/servo,mbrubeck/servo,dhananjay92/servo,GyrosOfWar/servo,dhananjay92/servo,runarberg/servo,jimberlage/servo,runarberg/servo,kindersung/servo,saratang/servo,ConnorGBrewster/servo,peterjoel/servo,huonw/servo,deokjinkim/servo,saneyuki/servo,tafia/servo,fiji-flo/servo,g-k/servo,s142857/servo,mdibaiee/servo,zentner-kyle/servo,ConnorGBrewster/servo,peterjoel/servo,j3parker/servo,vks/servo,dati91/servo,eddyb/servo,saratang/servo,runarberg/servo,fiji-flo/servo,dati91/servo,sadmansk/servo,paulrouget/servo,sadmansk/servo,nrc/servo,g-k/servo,tempbottle/servo,cbrewster/servo,pyecs/servo,jdramani/servo,boghison/servo,kindersung/servo,emilio/servo,dhananjay92/servo,rentongzhang/servo,mdibaiee/servo,mattnenterprise/servo,A-deLuna/servo,s142857/servo,GyrosOfWar/servo,jlegendary/servo,wartman4404/servo,dati91/servo,boghison/servo,notriddle/servo,aidanhs/servo,szeged/servo,dsandeephegde/servo,dati91/servo,akosel/servo,deokjinkim/servo,dhananjay92/servo,jdramani/servo,wartman4404/servo,brendandahl/servo,saratang/servo,anthgur/servo,DominoTree/servo,upsuper/servo,KiChjang/servo,szeged/servo,RenaudParis/servo,eddyb/servo,codemac/servo,paulrouget/servo