Fill in missing keyframe values.
Fill in missing keyframe values.

This is mostly a mimic of what we do in
GeckoCSSAnimationBuilder::FillInMissingKeyframeValues().

In Gecko we iterate over the properties just once because we can take the
index for both the synthesized start and end keyframe and easily look them up
as needed. However, in this patch we synthesize the start and end keyframes
separately and iterate over the properties twice because that's easier than
getting two indices and then later calling another FFI to dereference each of
them, and neater than getting back two pointers

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