style: Add a root bucket to the selector map.
style: Add a root bucket to the selector map.

:root can't change without getting unbound from the tree so no fancy stuff other
than that needed.

This removes a lot of revalidation and attribute invalidation matching from the
Chrome, and looks like it should be a good idea in general.

Differential Revision: https://phabricator.services.mozilla.com/D2462

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