style: Add a Servo API to get the serialized style of a property.
style: Add a Servo API to get the serialized style of a property.

This is intended to be used by GetComputedStyle when there's no layout
dependency.

Bug: 1467536
Reviewed-by: xidorn
MozReview-Commit-ID: 3GAbjo1uQ34

pyfisch/servo,notriddle/servo,splav/servo,saneyuki/servo,splav/servo,KiChjang/servo,sadmansk/servo,dati91/servo,larsbergstrom/servo,peterjoel/servo,sadmansk/servo,saneyuki/servo,notriddle/servo,larsbergstrom/servo,jimberlage/servo,DominoTree/servo,nnethercote/servo,nnethercote/servo,larsbergstrom/servo,szeged/servo,sadmansk/servo,notriddle/servo,paulrouget/servo,dati91/servo,pyfisch/servo,sadmansk/servo,splav/servo,emilio/servo,notriddle/servo,DominoTree/servo,pyfisch/servo,larsbergstrom/servo,splav/servo,nnethercote/servo,ConnorGBrewster/servo,cbrewster/servo,nnethercote/servo,KiChjang/servo,ConnorGBrewster/servo,KiChjang/servo,cbrewster/servo,jimberlage/servo,saneyuki/servo,peterjoel/servo,saneyuki/servo,peterjoel/servo,szeged/servo,saneyuki/servo,cbrewster/servo,dati91/servo,peterjoel/servo,pyfisch/servo,sadmansk/servo,jimberlage/servo,ConnorGBrewster/servo,saneyuki/servo,pyfisch/servo,notriddle/servo,DominoTree/servo,peterjoel/servo,nnethercote/servo,ConnorGBrewster/servo,szeged/servo,dati91/servo,szeged/servo,cbrewster/servo,splav/servo,nnethercote/servo,SimonSapin/servo,notriddle/servo,saneyuki/servo,emilio/servo,larsbergstrom/servo,paulrouget/servo,KiChjang/servo,pyfisch/servo,larsbergstrom/servo,larsbergstrom/servo,ConnorGBrewster/servo,DominoTree/servo,jimberlage/servo,emilio/servo,paulrouget/servo,paulrouget/servo,dati91/servo,KiChjang/servo,peterjoel/servo,splav/servo,SimonSapin/servo,KiChjang/servo,pyfisch/servo,peterjoel/servo,peterjoel/servo,paulrouget/servo,KiChjang/servo,saneyuki/servo,SimonSapin/servo,DominoTree/servo,jimberlage/servo,splav/servo,DominoTree/servo,szeged/servo,larsbergstrom/servo,dati91/servo,paulrouget/servo,cbrewster/servo,paulrouget/servo,jimberlage/servo,DominoTree/servo,splav/servo,SimonSapin/servo,DominoTree/servo,splav/servo,pyfisch/servo,dati91/servo,emilio/servo,notriddle/servo,szeged/servo,szeged/servo,jimberlage/servo,DominoTree/servo,SimonSapin/servo,SimonSapin/servo,nnethercote/servo,KiChjang/servo,pyfisch/servo,emilio/servo,nnethercote/servo,cbrewster/servo,larsbergstrom/servo,KiChjang/servo,DominoTree/servo,peterjoel/servo,emilio/servo,SimonSapin/servo,jimberlage/servo,jimberlage/servo,paulrouget/servo,nnethercote/servo,emilio/servo,peterjoel/servo,splav/servo,emilio/servo,notriddle/servo,ConnorGBrewster/servo,sadmansk/servo,nnethercote/servo,paulrouget/servo,cbrewster/servo,szeged/servo,saneyuki/servo,paulrouget/servo,sadmansk/servo,cbrewster/servo,saneyuki/servo,ConnorGBrewster/servo,KiChjang/servo,dati91/servo,SimonSapin/servo,sadmansk/servo,notriddle/servo,szeged/servo,notriddle/servo,larsbergstrom/servo,ConnorGBrewster/servo,pyfisch/servo,emilio/servo,emilio/servo,szeged/servo