Check that XMLHttpRequest.response returns null or a json object as
specified in the spec according to readyState value.

BUG=119256

Review URL: https://chromiumcodereview.appspot.com/23042006

git-svn-id: svn://svn.chromium.org/blink/trunk@156376 bbb929c8-8fbe-4397-9dbb-9b2b20218538