Bug 1053276 - [Messages] ActivityHandler tests are failing if the test a...

...gent is hidden r=azasypkin
2 answers for this:
* we have a new iframe for each test file, so yes :)
* and anyway, the "hidden" property is not in "document" but in the prototype chain; so calling "delete document.hidden" just makes the inherited property show up. You can try it yourself in devtools :)