Document.origin Status

According to recent test I've performed, Document.origin is not implemented
in any of Chrome, Firefox, Opera, or Safari. I haven't checked IE.

It is not marked as at-risk in the current CR. I also notice it isn't
tested in WPT [2].

In order to proceed to PR, this attribute needs to have appropriate tests
added to [2] and two implementations that support it.

[1] http://www.w3.org/TR/2014/CR-dom-20140508/
[2] https://github.com/w3c/web-platform-tests/tree/master/dom