Support for :enabled selector on Anchor & Area elements

Updates the PseudoEnabled selector checker to check for Anchor & Area
elements with a link. Reverts ef7563d45ca529252c4caa612dece0ddb1dcfc1d
which removed relevant Shadow DOM tests.

Spec: http://html.spec.whatwg.org/#selector-enabled

BUG=393105

Review URL: https://codereview.chromium.org/408293002

git-svn-id: svn://svn.chromium.org/blink/trunk@179427 bbb929c8-8fbe-4397-9dbb-9b2b20218538