Reland removing prefixed Shadow DOM APIs in Element.

Now that https://codereview.chromium.org/214093002/ has landed,
we can reland this safely.

BUG=336121

Review URL: https://codereview.chromium.org/216973003

git-svn-id: svn://svn.chromium.org/blink/trunk@170374 bbb929c8-8fbe-4397-9dbb-9b2b20218538