Bug 837678 - [cost control] Implement new startup loading events

What @lodr is saying sounds correct. We want to trigger `moz-chrome-dom-loaded` at the moment when the app chrome UI elements have been marked for display, whether that be by removing a class name, removing `display: none`, etc. At the point where they are then displayed is where that event should be triggered.

The same goes for `moz-app-visually-complete`, except the elements we are referring to are different. This event should be triggered when the app's core interface or primary content are visible.