Dirty tree scopes are always a subset of active tree scopes.

No need to walk dirty tree scopes when clearing media dependent rule
sets after walking the active tree scopes.

Also clear m_dirtyTreeScopes in StyleEngine::detachFromDocument.
Currently, m_dirtyTreeScopes are synchronously updated and cleared
right after they are marked dirty, but that is supposed to change.

BUG=567021

Review URL: https://codereview.chromium.org/1786663003

Cr-Commit-Position: refs/heads/master@{#381909}
1 file changed