Remove redundant checks from SharedStyleFinder::canShareStyleWithElement

Many of the checks in canShareStyleWithElement are not needed because
Element::supportsStyleSharing checks the same thing and call that before putting
the element into the candidate list. This patch removes all the redundant checks
and switches parent to be a reference since we know the candidate will always have
a parent element from the check in supportsStyleSharing.

Review URL: https://codereview.chromium.org/32753006

git-svn-id: svn://svn.chromium.org/blink/trunk@160236 bbb929c8-8fbe-4397-9dbb-9b2b20218538