Unguard Shadow DOM features.

Runtime ShadowDOM flag was already set to true at r169201. This change shouldn't have any effects.

CSSSelector still uses this flag. I'll clean up CSSSelector in another patch.

BUG=336121

Review URL: https://codereview.chromium.org/206033004

git-svn-id: svn://svn.chromium.org/blink/trunk@169912 bbb929c8-8fbe-4397-9dbb-9b2b20218538