Bug 982663: Collect and report app usage data

https://bugzilla.mozilla.org/show_bug.cgi?id=982663
Should we have a bootstrap process for the usage metrics?
I mean, instead of loading the whole usage library, we could just have a tiny file that in case that `debug.performance_data.shared` is enable we lazy load the rest of the library and use it.

Basically, getting the `start` method out, and lazyloading depending on the setting value.