Pre-bind the 'xml' prefix for serialization of XML fragments

The Namespaces in XML spec states:

"The prefix xml is by definition bound to the namespace name
http://www.w3.org/XML/1998/namespace."

BUG=340794

Review URL: https://codereview.chromium.org/183663033

git-svn-id: svn://svn.chromium.org/blink/trunk@168673 bbb929c8-8fbe-4397-9dbb-9b2b20218538