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.
nit: group the `var` declarations together, e.g.

```js
var var memUsage = performanceHelper.getMemoryUsage(app);
var start = runResults.start || 0;

app.close();
// ...
```