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.
@walac & @hfiguiere: Why are we listening for the `startup-path-done` event and not just `moz-app-loaded`? Haven't all apps been updated to emit `moz-app-loaded`?