[DOM4] Doctypes now always have a node document and can be moved across document boundaries

Doctypes now always have a node document and can be moved across document boundaries as per
the latest DOM4 specification:
http://dom.spec.whatwg.org/#dom-domimplementation-createdocumenttype
http://dom.spec.whatwg.org/#dom-node-ownerdocument

This means that DOMImplementation.createDocumentType() now sets the ownerDocument of the
new DocumentType Node to the associated document of the current "context" object. In
DOM4, all nodes have a document at all times. DocumentType nodes can now be moved across
document boundaries so that the node can be added to a Document after being created.

Firefox stable already follows DOM4 here while IE10 does not (yet).

BUG=275600

Review URL: https://chromiumcodereview.appspot.com/22880019

git-svn-id: svn://svn.chromium.org/blink/trunk@156581 bbb929c8-8fbe-4397-9dbb-9b2b20218538