Revert "Revert "Implement Element.matches(selectors)""

This re-lands <https://codereview.chromium.org/106983004>, which was
reverted in <https://codereview.chromium.org/103303005>.

The root bug causing the revert was fixed in Polymer in
<https://github.com/Polymer/ShadowDOM/pull/332> and reached Chromium in
<https://codereview.chromium.org/98583008>.

BUG=326652

Review URL: https://codereview.chromium.org/133643006

git-svn-id: svn://svn.chromium.org/blink/trunk@164970 bbb929c8-8fbe-4397-9dbb-9b2b20218538