Revert of Support for :enabled selector on Anchor & Area elements (patchset #2 id:20001 of https://codereview.chromium.org/408293002/)

Reason for revert:
HTML spec has been modified [1] to disable support for :enabled CSS selector on Anchor, Area & Link elements, after discussion on W3C Bugzilla [2].

[1] https://html5.org/r/8818
[2] https://www.w3.org/Bugs/Public/show_bug.cgi?id=26622

Original issue's description:
> 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
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=179427

TBR=esprehn@chromium.org,alexis.menard@intel.com,dglazkov@chromium.org,jchaffraix@chromium.org
BUG=393105

Review URL: https://codereview.chromium.org/610683002

git-svn-id: svn://svn.chromium.org/blink/trunk@182932 bbb929c8-8fbe-4397-9dbb-9b2b20218538