Implement cloneNode for Document

When cloning HTMLDocument and SVGDocument we need to ensure that we
create a new HTMLDocument and SVGDocument respectively.

http://dom.spec.whatwg.org/#dom-node-clonenode

BUG=258146

Review URL: https://chromiumcodereview.appspot.com/22622002

git-svn-id: svn://svn.chromium.org/blink/trunk@155787 bbb929c8-8fbe-4397-9dbb-9b2b20218538