Make several DOMImplementation API arguments mandatory

Make the arguments of the following DOMImplementation API arguments mandatory:
- createDocumentType()
- createDocument()

This matches the specification:
http://dom.spec.whatwg.org/#interface-domimplementation

The arguments are already mandatory in Firefox and IE10 so the compatibility
risk should be low.

BUG=272969

Review URL: https://chromiumcodereview.appspot.com/22875013

git-svn-id: svn://svn.chromium.org/blink/trunk@156303 bbb929c8-8fbe-4397-9dbb-9b2b20218538