Implement Style Containment

The only CSS feature we support that is bound by style containment is
counters. This patch adds the concept of an enclosing style containment
ancestor to the tree walking logic in LayoutCounter and avoids crossing
these boundaries when matching counters.

Layout Containment description from spec:

Intent to implement:!topic/blink-dev/9W80Kw-z3ss

Review URL:

Cr-Commit-Position: refs/heads/master@{#375684}
7 files changed