SelectorChecker should traverse from the element when :host is found.

To make :host's parameter match a shadow host which matches :host, need to start from the element,
not its parent element.

BUG=313935

TEST=fast/dom/shadow/pseudo-host-parameter-matches-shadow-host.html

Review URL: https://codereview.chromium.org/55693002

git-svn-id: svn://svn.chromium.org/blink/trunk@161190 bbb929c8-8fbe-4397-9dbb-9b2b20218538