Use underlying value for missing keyframes.
Use underlying value for missing keyframes.

mBaseStyleValuesForServo is a nsRefPtrHashtable<nsUint32HashKey, RawServoAnimationValue>
In this patch, we use void* instead of exposing nsRefPtrHashtable in FFI
because we just use the hash table as an argument of a C++ function, it means
we don't touch the hash table in Rust at all.

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