Bug 1104927 - UITour: Update browser_UITour_loop.js with for Loop:ChatWindowShown. r=me
Bug 1106584, Part 1: guard against null intent. r=vng
Bug 1106584 - Part 2: for safety, make explicit prefs with context getter. r=vng

Any class that has access to a context should call Prefs.getInstance(Context) which is guaranteed to return a Prefs.
With no context, classes can call Prefs.getInstanceWithoutContext() but they must null-check the return, and handle accordingly.
Fortunately, the latter case happens in very few places, all of which require no special handling of this case.

This change:
- maintains context-less access to the Prefs
- classes internal to the service can safely call getInstanceWithoutContext(), as the service (or the MainApp in the stumbler case)
will have instantiated a Prefs so that getInstanceWithoutContext() will return a prefs
- protects against null Prefs if we have failed to consider a particular entry point to the code will require that Prefs was instatiated with a context.
Bug 1109233 - Replaced Assert.isTrue(false, ...) with Assert.fail(...) for easier readability; r=mcomella