style: Use nsAtom instead of nsString for FontFamilyName.
style: Use nsAtom instead of nsString for FontFamilyName.

Bug: 1490997
Reviewed-by: lsalzman

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