Remove Runtime ShadowDOM flag.

This patch also cleans up CSSSelector so that it doesn't use ShadowDOM flag.
Runtime ShadowDOM flag was already set to true at r169201. This change shouldn't have any effects.

BUG=336121

Review URL: https://codereview.chromium.org/210763002

git-svn-id: svn://svn.chromium.org/blink/trunk@170009 bbb929c8-8fbe-4397-9dbb-9b2b20218538