Bug 1044297 - Update memory performance tests to use new moz-app-loaded ...

...event.

After moz-app-loaded event, application has displayed content and ready
for interation. At this point, we should check for memory consumption
regressions.
Please see https://github.com/mozilla-b2g/gaia/blob/master/tests/performance/startup_test.js#L84

You are missing the check that we actually have enough data requested.