SelectorChecker::parentElement should walk up to ShadowHost only if an element is in a shadow tree hosted by the ShadowHost and scope is the ShadowHost.

:host can only match shadow host. :host depends on context.scope with ScopeIsShadowHost behavior.
So only if context.scope is shadow host and context.element's shadow host is context.scope, we need to check the shadow host.

BUG=370303
TEST=fast/dom/shadow/style-with-cat.html, fast/dom/shadow/style-with-hat.html

Review URL: https://codereview.chromium.org/273523008

git-svn-id: svn://svn.chromium.org/blink/trunk@173703 bbb929c8-8fbe-4397-9dbb-9b2b20218538