Implemented DocumentType.cloneNode(bool)

Spec: http://dom.spec.whatwg.org/#concept-node-clone

BUG=325226

Review URL: https://codereview.chromium.org/102533002

git-svn-id: svn://svn.chromium.org/blink/trunk@164168 bbb929c8-8fbe-4397-9dbb-9b2b20218538