Use ScopeIsShadowHost when element is in descentant tree of shadow host of scoping node.

If a given element is a descendant node of scoping node, we should not treat the scoping node as shadow host.
However, when the element is distributed and selector has ::content, we need to check the scoping node again,
because the element is moved into a descendant tree hosted by the scoping node(=shadow host).

BUG=313941
TEST=fast/dom/shadow/style-with-cat.html,fast/dom/shadow/content-pseudo-element-with-hat.html,fast/dom/shadow/content-pseudo-element-with-cat.html

Review URL: https://codereview.chromium.org/61033005

git-svn-id: svn://svn.chromium.org/blink/trunk@161759 bbb929c8-8fbe-4397-9dbb-9b2b20218538