Expose URLUtilsReadOnly.origin

Follow up on a recent URL spec addition,

 http://url.spec.whatwg.org/#urlutilsreadonly
 https://github.com/whatwg/url/commit/0027a2e

and add the |origin| property to URLUtilsReadOnly.

WorkerLocation is currently the only object implementing this interface,
so this addition lets scripts executing in Worker contexts look up the
serialized origin (of the script that initialized the Worker) via
self.location.origin.

R=
BUG=303152

Review URL: https://codereview.chromium.org/342553010

git-svn-id: svn://svn.chromium.org/blink/trunk@176712 bbb929c8-8fbe-4397-9dbb-9b2b20218538