Bug 1090087 - Add test for saving an image from a webpage

If I use the Activities object from the system region it will fail in the [init]. That is because ```location['y'] == 0```. There is also an activities file in the [messages folder][link] and a context_menu file in the [homescreen folder][link2].

How do you propose I should handle this?

[init]:https://github.com/mozilla-b2g/gaia/blob/master/tests/python/gaia-ui-tests/gaiatest/apps/system/regions/activities.py#L26
[link]:https://github.com/mozilla-b2g/gaia/blob/master/tests/python/gaia-ui-tests/gaiatest/apps/messages/regions/activities.py
[link2]:https://github.com/mozilla-b2g/gaia/blob/master/tests/python/gaia-ui-tests/gaiatest/apps/homescreen/regions/context_menu.py