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.
I know it is cut and paste, but then this is not in alphabetic order.

Also I think we could make that common with startup_event_test.js as part of a small refactoring. Actually it should be done now. You previous iteration did that.