CSS: Implement invalidation for ::part

This adds an understanding of ::part to InvalidationSet.

Changes RuleFeatureSet to set invalidates_parts and
tree_boundary_crossing for ::part selectors.

Bug: 805271
Change-Id: I905b7f5a700acfc08bd9a01d4cda12a0de4eac18
Reviewed-on: https://chromium-review.googlesource.com/1068299
Commit-Queue: Fergal Daly <fergal@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561435}
11 files changed