Move TreeBoundaryCrossingScopes to StyleEngine.
This is split out of the work for async stylesheet updates [1], but is
also part of the work on componentized style resolving in general.
The moved resetAuthorStyle method on StyleEngine may soon be gone
altogether as it does so in [1].
The plan is that TreeBoundaryCrossingScopes will also be completely
gone when we remove support for Shadow DOM v0. For Shadow DOM v1 we can
look up the scoped resolvers for the affecting scopes directly like we
already do in StyleResolver::matchScopedRules for the pure v1 case.
The documentation of the special casing of VTT and custom pseudo
elements is updated to not suggest that these rules are handled as part
of boundary crossing scopes as the current solution is better once v0
shadows go away.
[1] https://codereview.chromium.org/1913833002
R=meade@chromium.org
BUG=567021,401359
Review-Url: https://codereview.chromium.org/2443933002
Cr-Commit-Position: refs/heads/master@{#427284}
8 files changed
