Remove Internals::ensureShadowRoot(Element)

Internals::ensureShadowRoot(Element) is an old internal API.
We can rewrite existing layout tests so that they don't use this internal API anymore.

Review URL: https://codereview.chromium.org/146213005

git-svn-id: svn://svn.chromium.org/blink/trunk@166641 bbb929c8-8fbe-4397-9dbb-9b2b20218538