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

These event "startup-path-done" are now deprecated. You should use either ````window.dispatchEvent(new CustomEvent('moz-app-visually-complete'))```` or ````window.dispatchEvent(new CustomEvent('moz-chrome-interactive'))```` depending on the case.

See https://developer.mozilla.org/en-US/Apps/Build/Performance/Firefox_OS_app_responsiveness_guidelines