Move supportsStyleSharing to Element

This will let us use some private/protected APIs in Element or Node to
skip some of the branching in the future and also removes null checks that
were impossible to hit.

Review URL: https://codereview.chromium.org/26897004

git-svn-id: svn://svn.chromium.org/blink/trunk@159921 bbb929c8-8fbe-4397-9dbb-9b2b20218538