The first part just handles the case where nsAudioStream failed to allocate a stream. It won't be playing
anything, so instead of trying to get the audio position, just fall back to the media graph current time.
Otherwise GetPositionInFrames returns -1 and things go badly from there.
The second part simplifies the calculation of the next mCurrentTime to just make it based on real time.
We had some code to not let it advance past the end of a stream's buffer, but the next part will make that
unnecessary.
The third part is the real fix. When the new current time has advanced past mBlockingDecisionsMadeUntilTime,
that means the control loop didn't run in time to replenish the audio output buffers and keep up with its
other duties. Effectively all streams have been blocked between mBlockingDecisionsMadeUntilTime and
the new current time. Account for that by adding the difference as extra blocked time for every stream.
We only need to ensure that the stream is marked blocked from mBlockingDecisionsMadeUntilTime indefinitely
far into the future, and then update mBlockingDecisionsMadeUntilTime to the new current time, because the
code takes into account that only blocking decisions up to mBlockingDecisionsMadeUntilTime are valid.
