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.
You don't check the number of runs here. Just that we have some results.
````    assert.ok(results.length == config.runs, 'missing runs');````