style: Deduplicate system metric atoms.
style: Deduplicate system metric atoms.

Now that :-moz-system-metric is gone, there's no real reason for the atoms to
be separate.

Differential Revision: https://phabricator.services.mozilla.com/D3497

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