Use ScopeIsShadowHost when collecting and matching rules from ::content rulesets.

BUG=274063

TEST=fast/dom/shadow/content-pseudo-element-with-host-pseudo-class.html

Review URL: https://codereview.chromium.org/22978026

git-svn-id: svn://svn.chromium.org/blink/trunk@158705 bbb929c8-8fbe-4397-9dbb-9b2b20218538