Add support for DOM4's XMLDocument interface

Add support for DOM4's XMLDocument interface:
http://dom.spec.whatwg.org/#xmldocument

DOMImplementation.createDocument() now returns an XMLDocument instead of a
Document as per:
http://dom.spec.whatwg.org/#dom-domimplementation-createdocument

DOMParser.parseFromString() now returns an XMLDocument instead of a Document
when the input type is an xml type, as per:
http://domparsing.spec.whatwg.org/#dom-domparser-parsefromstring

This behavior is consistent with Firefox 26 and IE 11.
The compatibility risk is low since an XMLDocument is identical to a Document.

R=tkent, arv
BUG=238366
BUG=238372
TEST=fast/dom/DOMImplementation/createDocument-XMLDocument.html

Review URL: https://codereview.chromium.org/144063004

git-svn-id: svn://svn.chromium.org/blink/trunk@165607 bbb929c8-8fbe-4397-9dbb-9b2b20218538