Bug 1087042 - Rocketbar fullscreen-app r=kgrandon

Asserting here is tricky as I feel that this may create an intermittent test. The problem is that the page may be fully loaded before the assertion. The only way to not make this intermittent is to use a fake HTTP server and stop it from loading. If this does pass gaia-try, and you don't want to address this here, then please add a comment about how this may be intermittent in case someone has to investigate it in the future.

https://github.com/mozilla-b2g/gaia/blob/master/apps/verticalhome/test/marionette/server/parent.js#L129
https://github.com/mozilla-b2g/gaia/blob/master/apps/verticalhome/test/marionette/server/child.js#L252