Make ScriptTask::js_context immutable.
Make ScriptTask::js_context immutable.

Since we drop the ScriptTask almost immediately after clearing the field,
there's little reason to do it manually.

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