OS X 10.9 (Mavericks) introduced a hint (kAudioHardwarePowerHintFavorSavingPower) to opt-in to power savings in the CoreAudio stack. It may be that we already get most of the benefit by setting the I/O buffer size to a large value when creating a high latency stream. The hint is a global property, so we need to have explicit logic to set and unset it depending on what streams are active, i.e. we'd want kAudioHardwarePowerHintFavorSavingPower only when high latency streams are playing, and kAudioHardwarePowerHintNone if any low latency streams are active (this probably also applies to the existing logic to set the I/O buffer size, as I think a later high latency stream will clobber the setup for an existing low latency stream otherwise).

I think it's better for this bug to block the general Mavericks compatibility bug. Bug 930586 frankly doesn't have much content (though it took me a while to realize that).

Mass change P2->P3 to align with new Mozilla triage process.