Add default value to layout.threads in prefs.rs (fix15638)
Add default value to layout.threads in prefs.rs (fix15638)

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