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

third_party/WebKit/Source/platform/RuntimeEnabledFeatures.in

@@ -99,7 +99,6 @@ ScrollTopLeftInterop status=experimental
 
 ServiceWorker status=experimental
 SessionStorage status=stable
-ShadowDOM status=stable
 SharedWorker status=stable
 SpeechInput status=stable
 SpeechSynthesis status=stable