Remove mutability requirement from fetch_async_background.
Remove mutability requirement from fetch_async_background.

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