Remove some checks from SharedStyleFinder

Element::supportsStyleSharing() always returns false for elements that
are hovered, focused or active or are the :target so we don't need to
check those in the SharedStyleFinder.

Review URL: https://codereview.chromium.org/221463004

git-svn-id: svn://svn.chromium.org/blink/trunk@170613 bbb929c8-8fbe-4397-9dbb-9b2b20218538