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When crossing an insertion point boundary from style recalc, we mark the
distributed nodes for subtree style recalc. The reason is that the
distributed nodes will have their style recalculated in the context of
where they are distributed from. In the case below, changing class=a on
content will affect the style of class=b. The invalidation starts on
class=a and will not reach class=b in the invalidation traversal.

The bug was that InsertionPoint::willRecalcStyle had an early return
if the parent style change was at least an Inherit, but that is not the
case if the class change does not affect anything at, or above, the
insertion point, like in the case below.

<div>
  <:shadow-root>
    <style>.a::content .b { ... }</style>
    <content class="a"></content>
  </:shadow-root>
  <div class="b"></div>
</div>

R=esprehn@chromium.org,chrishtr@chromium.org
BUG=423293

Review URL: https://codereview.chromium.org/637273003

git-svn-id: svn://svn.chromium.org/blink/trunk@183844 bbb929c8-8fbe-4397-9dbb-9b2b20218538