Test that substituting something different to window.JSON doesn't break XMLHttpRequest

Clarified on the WHATWG version.
http://xhr.spec.whatwg.org/#json-response-entity-body

BUG=none

Review URL: https://codereview.chromium.org/27692002

git-svn-id: svn://svn.chromium.org/blink/trunk@160181 bbb929c8-8fbe-4397-9dbb-9b2b20218538