Convert XMLDocumentParser code to Use more reference

Document pointer can not be null to create XMLDocumentParser.
Move the parameter to use reference to make caller code safer.

Review URL: https://codereview.chromium.org/276733002

git-svn-id: svn://svn.chromium.org/blink/trunk@173780 bbb929c8-8fbe-4397-9dbb-9b2b20218538