Audio output on macOS was consuming Core Audio resources until the
PlaybackStream creation took well over the timeout for some tests.
This was observed in media-source-buffered.html, where it would time
out due to the long-running callback on the main thread to create the
PlaybackStream for AudioMixingSink.
However, the AudioUnit init should definitely not be blocking the main
thread, so I've added a FIXME there.
1467127d35