style: Add support for the 'content' CSS property on ::marker pseudo elements.
style: Add support for the 'content' CSS property on ::marker pseudo elements.

Bug: 205202
Reviewed-by: emilio

paulrouget/servo,notriddle/servo,saneyuki/servo,DominoTree/servo,larsbergstrom/servo,saneyuki/servo,KiChjang/servo,larsbergstrom/servo,DominoTree/servo,pyfisch/servo,splav/servo,pyfisch/servo,pyfisch/servo,splav/servo,splav/servo,DominoTree/servo,larsbergstrom/servo,notriddle/servo,KiChjang/servo,pyfisch/servo,larsbergstrom/servo,splav/servo,saneyuki/servo,emilio/servo,KiChjang/servo,larsbergstrom/servo,saneyuki/servo,notriddle/servo,emilio/servo,nnethercote/servo,splav/servo,nnethercote/servo,emilio/servo,DominoTree/servo,paulrouget/servo,larsbergstrom/servo,paulrouget/servo,splav/servo,splav/servo,KiChjang/servo,paulrouget/servo,KiChjang/servo,DominoTree/servo,notriddle/servo,saneyuki/servo,nnethercote/servo,saneyuki/servo,paulrouget/servo,nnethercote/servo,larsbergstrom/servo,emilio/servo,pyfisch/servo,larsbergstrom/servo,paulrouget/servo,nnethercote/servo,emilio/servo,saneyuki/servo,notriddle/servo,splav/servo,KiChjang/servo,DominoTree/servo,nnethercote/servo,nnethercote/servo,pyfisch/servo,pyfisch/servo,notriddle/servo,nnethercote/servo,KiChjang/servo,saneyuki/servo,notriddle/servo,KiChjang/servo,paulrouget/servo,pyfisch/servo,emilio/servo,KiChjang/servo,notriddle/servo,DominoTree/servo,DominoTree/servo,saneyuki/servo,paulrouget/servo,KiChjang/servo,larsbergstrom/servo,emilio/servo,notriddle/servo,nnethercote/servo,pyfisch/servo,emilio/servo,splav/servo,DominoTree/servo,paulrouget/servo,nnethercote/servo,DominoTree/servo,pyfisch/servo,notriddle/servo,emilio/servo,splav/servo,emilio/servo,larsbergstrom/servo,saneyuki/servo,paulrouget/servo