Invalidate custom pseudo elements for RuleSet invalidations.
When we have selectors containing custom pseudo elements matching
elements inside the UA shadow, and no id, class, or attribute selectors
present, do like we do for type selectors and invalidate as part of an
invalidation set scheduled on the root node of the tree scope for
We utilize the same invalidation set as for type invalidations and mark
it as invalidating custom pseudo elements as well as marking it as
tree-boundary-crossing to allow drilling into the UA shadow.
This means we will traverse into all shadow sub-trees, but it should at
least be better than the existing recalc all behavior.
The full recalc for custom pseudo elements caused a full recalc for one
of the stylesheets on facebook.com