Bug 815542 - crash in IncrementalCollectSlice r=billm

--HG--
extra : rebase_source : d751b3684f6461b094278aa39508417928c2b660
Bug 831628 - 0001. Add event object. r=yoshi. sr=sicking
Bug 831628 - 0002. Handle event. r=yoshi
Bug 831628 - 0003. Mark event as supported. r=yoshi
Bug 831628 - 0004. Change worker name. r=yoshi
Bug 831628 - 0005. Test case for Event Download Command : Idle Screen Available. r=yoshi