Bug 952893. Part 1: Block the AudioDestinationNode when it's the only node in the AudioContext. r=padenot,karlt
Bug 952893. Part 2: When all streams in the MSG are blocked, allow the MSG to suspend indefinitely and pause AudioStream outputs while suspended. r=padenot
Bug 952893. Part 3: Tighten TimeVarying invariants so that GetAt always returns an end time when the value actually changes. r=padenot