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For shadow dom v0, we used a SubtreeStyleChange for propagating
inherited style changes through insertion points to distributed nodes.
LocalStyleChange should suffice. We already use LocalStyleChange in the
HTMLSlotElement case.

We still need to use SubtreeStyleChange where we have a
SubtreeStyleChange/Force from further up the tree like:

<host>
  <:shadow-root>
    <style>.a::content * { background: green }</style>
    <div id="a">
      <content></content>
    </div>
  </:shadow-root>
  <div>Green when #a gets class a.</div>
</host>

R=kochi@chromium.org
BUG=638869

Review-Url: https://codereview.chromium.org/2258793003
Cr-Commit-Position: refs/heads/master@{#414386}