Node.contains should return true for nodes in Shadow DOM.

This is discussed on this W3 bug:
<https://www.w3.org/Bugs/Public/show_bug.cgi?id=22141>

BUG=265697

Review URL: https://chromiumcodereview.appspot.com/21123005

git-svn-id: svn://svn.chromium.org/blink/trunk@155161 bbb929c8-8fbe-4397-9dbb-9b2b20218538