Have XMLHttpRequest.responseXML return an XMLDocument

Have XMLHttpRequest.responseXML return an XMLDocument instead of a Document
when the response is of XML MIME type. This behavior is consistent with
Firefox 27 and IE11.

R=arv, tkent
BUG=350923
TEST=http/tests/xmlhttprequest/responsexml-type.html

Review URL: https://codereview.chromium.org/197103005

git-svn-id: svn://svn.chromium.org/blink/trunk@169080 bbb929c8-8fbe-4397-9dbb-9b2b20218538