Bug 850587. Part 1: Refactor NotifyAutoplayDataReady to CheckAutoplayDataReady and allow autoplay to start on HAVE_CURRENT_DATA if the source is a stream. r=cpearce,a=baja
Bug 850587. Part 2: Make NotifyHasCurrentData fire when a stream would be able to produce data if it was not blocked. r=jesup,a=bajaj
Also removes NotifyHasCurrentData's boolean parameter; we just fire this
once and treat a stream that has once had current data as always
having current data (since we block a stream that would advance
beyond its available data).