Don't special-case @region subselectors in scoped stylesheets.

This was done in r160815. I don't think it was correct. I think
we can use fast checkable selectors in @region subselectors in
scoped stylesheets just as we can with normal selectors in
scoped stylesheets.

Review URL: https://codereview.chromium.org/65513002

git-svn-id: svn://svn.chromium.org/blink/trunk@161563 bbb929c8-8fbe-4397-9dbb-9b2b20218538