Don't expose WebKitShadowRoot constructor.

It's time to expose only ShadowRoot constructor.
The chromium side patch is here: https://codereview.chromium.org/217013005/.

BUG=242153

Review URL: https://codereview.chromium.org/216123005

git-svn-id: svn://svn.chromium.org/blink/trunk@170580 bbb929c8-8fbe-4397-9dbb-9b2b20218538