Use Reflect on attributes in xml namespace

Use Reflect on attributes in xml namespace, in order make this work teach v8_attributes.py about XMLNames namespace.

Review URL: https://codereview.chromium.org/378343004

git-svn-id: svn://svn.chromium.org/blink/trunk@177847 bbb929c8-8fbe-4397-9dbb-9b2b20218538