Auto merge of #18885 - servo:unstable-cef, r=nox
Auto merge of #18885 - servo:unstable-cef, r=nox

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.

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