Bug 837675 - [music] Implement new startup loading events

It would be pretty easy to determine a resasonable upper bound for the number of tiles that fit, or you could compute this based on the known size of the tiles and window.innerHeight.   But I'd be okay if you just sent the events after 12 tiles (or when the last tile is processed, whichever comes first).

moz-content-interactive is the one that is going to be graphed in datazilla, isn't it? If so, it seems like deferring this will lead to performance numbers that will degrade with large music collections.  On the other hand, the fact that the code uses enumerateAll() may mean that that performance price has already been paid anyway.