Add a default 'unstable' feature to CEF
Add a default 'unstable' feature to CEF

I recently moved that default from libservo to ports/servo,
which made CEF not have it anymore.

The different defaults made `./mach build-cef` after `./mach build`
rebuild many libraries (including serde and all of its recursive dependents)
and take a long time.

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