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multi-channel audio.
Moving the device opening into the audio thread will prevent browser hangs for badly behaving alsa implementations (like pulseaudio) that can hang snd_pcm_open if pulseaudiod is wedged, even if SND_PCM_NONBLOCK is requested.
For multi-channel audio, device enumeration has been added to try and find a multi-channel device with a stable channel mapping. According to http://0pointer.de/blog/projects/guide-to-sound-apis.html, default should only be used with mono and stereo stream because the channel ordering is not defined by Alsa. To get a well-defined channel ordering, one must use one of the surround40, surround51, etc., device names. However, these device names do not always allow multiple opens, so a fallback scheme is implemented to use default if necessary.
BUG=20945,17703
TEST=listened with built-in soundcard and USB soundcard with various other audio programs running.
Review URL: http://codereview.chromium.org/275022
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@29299 0039d316-1c4b-4281-b951-d872f2087c98
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