Dirty tree scopes are always a subset of active tree scopes.
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: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#381909}

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