Auto merge of #11918 - pcwalton:wr-no-paint-threads, r=glennw
Auto merge of #11918 - pcwalton:wr-no-paint-threads, r=glennw

gfx: Don't create paint threads when WebRender is in use.

They're unused and just waste memory and process table entries.

r? @glennw

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CJ8664/servo,nrc/servo,rnestler/servo,nrc/servo,sadmansk/servo,pyfisch/servo,notriddle/servo,upsuper/servo,DominoTree/servo,larsbergstrom/servo,emilio/servo,cbrewster/servo,peterjoel/servo,emilio/servo,SimonSapin/servo,nnethercote/servo,splav/servo,mbrubeck/servo,emilio/servo,upsuper/servo,pyfisch/servo,mbrubeck/servo,mattnenterprise/servo,SimonSapin/servo,notriddle/servo,peterjoel/servo,peterjoel/servo,dsandeephegde/servo,szeged/servo,ConnorGBrewster/servo,cbrewster/servo,KiChjang/servo,mbrubeck/servo,emilio/servo,eddyb/servo,sadmansk/servo,upsuper/servo,dati91/servo,canaltinova/servo,notriddle/servo,upsuper/servo,saneyuki/servo,nrc/servo,dati91/servo,dsandeephegde/servo,cbrewster/servo,nnethercote/servo,eddyb/servo,cbrewster/servo,szeged/servo,DominoTree/servo,mbrubeck/servo,notriddle/servo,CJ8664/servo,larsbergstrom/servo,canaltinova/servo,fiji-flo/servo,paulrouget/servo,fiji-flo/servo,splav/servo,paulrouget/servo,splav/servo,szeged/servo,thiagopnts/servo,fiji-flo/servo,dati91/servo,jimberlage/servo,anthgur/servo,larsbergstrom/servo,rnestler/servo,saneyuki/servo,splav/servo,KiChjang/servo,KiChjang/servo,eddyb/servo,ConnorGBrewster/servo,sadmansk/servo,DominoTree/servo,nnethercote/servo,mattnenterprise/servo,dati91/servo,nrc/servo,paulrouget/servo,emilio/servo,paulrouget/servo,jimberlage/servo,SimonSapin/servo,thiagopnts/servo,splav/servo,peterjoel/servo,KiChjang/servo,DominoTree/servo,splav/servo,KiChjang/servo,pyfisch/servo,eddyb/servo,notriddle/servo,thiagopnts/servo,szeged/servo,emilio/servo,saneyuki/servo,sadmansk/servo,mattnenterprise/servo,CJ8664/servo,peterjoel/servo,larsbergstrom/servo,upsuper/servo,DominoTree/servo,paulrouget/servo,saneyuki/servo,KiChjang/servo,upsuper/servo,thiagopnts/servo,notriddle/servo,avadacatavra/servo,fiji-flo/servo,CJ8664/servo,pyfisch/servo,fiji-flo/servo,fiji-flo/servo,thiagopnts/servo,rnestler/servo,cbrewster/servo,ConnorGBrewster/servo,DominoTree/servo,saneyuki/servo,peterjoel/servo,splav/servo,nnethercote/servo,notriddle/servo,fiji-flo/servo,jimberlage/servo,upsuper/servo,cbrewster/servo,pyfisch/servo,nnethercote/servo,mattnenterprise/servo,ConnorGBrewster/servo,peterjoel/servo,peterjoel/servo,larsbergstrom/servo,szeged/servo,SimonSapin/servo,SimonSapin/servo,dati91/servo,splav/servo,paulrouget/servo,thiagopnts/servo,jimberlage/servo,dsandeephegde/servo,dsandeephegde/servo,szeged/servo,anthgur/servo,SimonSapin/servo,emilio/servo,fiji-flo/servo,avadacatavra/servo,saneyuki/servo,eddyb/servo,DominoTree/servo,SimonSapin/servo,mattnenterprise/servo,pyfisch/servo,eddyb/servo,szeged/servo,rnestler/servo,sadmansk/servo,thiagopnts/servo,ConnorGBrewster/servo,mattnenterprise/servo,mattnenterprise/servo,cbrewster/servo,emilio/servo,dati91/servo,rnestler/servo,KiChjang/servo,paulrouget/servo,mbrubeck/servo,canaltinova/servo,larsbergstrom/servo,pyfisch/servo,pyfisch/servo,avadacatavra/servo,nnethercote/servo,sadmansk/servo,mbrubeck/servo,emilio/servo,thiagopnts/servo,splav/servo,saneyuki/servo,notriddle/servo,mattnenterprise/servo,jimberlage/servo,avadacatavra/servo,rnestler/servo,anthgur/servo,nnethercote/servo,larsbergstrom/servo,nrc/servo,larsbergstrom/servo,dati91/servo,ConnorGBrewster/servo,jimberlage/servo,paulrouget/servo,eddyb/servo,notriddle/servo,canaltinova/servo,sadmansk/servo,cbrewster/servo,anthgur/servo,splav/servo,nnethercote/servo,dsandeephegde/servo,peterjoel/servo,anthgur/servo,CJ8664/servo,anthgur/servo,szeged/servo,paulrouget/servo,paulrouget/servo,avadacatavra/servo,avadacatavra/servo,canaltinova/servo,larsbergstrom/servo,rnestler/servo,canaltinova/servo,upsuper/servo,nrc/servo,KiChjang/servo,mbrubeck/servo,nrc/servo,pyfisch/servo,jimberlage/servo,DominoTree/servo,anthgur/servo,saneyuki/servo,emilio/servo,avadacatavra/servo,dsandeephegde/servo,CJ8664/servo,SimonSapin/servo,larsbergstrom/servo,CJ8664/servo,KiChjang/servo,dati91/servo,DominoTree/servo,eddyb/servo,rnestler/servo,canaltinova/servo,anthgur/servo,dsandeephegde/servo,nnethercote/servo,mbrubeck/servo,jimberlage/servo,peterjoel/servo,szeged/servo,notriddle/servo,saneyuki/servo,ConnorGBrewster/servo,ConnorGBrewster/servo,DominoTree/servo,nnethercote/servo,pyfisch/servo,canaltinova/servo,jimberlage/servo,saneyuki/servo,avadacatavra/servo,dsandeephegde/servo,sadmansk/servo,CJ8664/servo,szeged/servo,KiChjang/servo