Get PulseAudio implementation working.

- Switches from iterating the PA mainloop to a threaded mainloop
which PulseAudio runs on its own.
- Fixes several bugs and cleans up the code.

Performance is much better than ALSA. I'm able to run audio
output at 440 frames smoothly with Pulse whereas ALSA I
couldn't get it to run below 1k w/o glitching.

Sadly while the jitter is much better too, it's still not
as smooth as the 8k buffer we were using previously:
http://commondatastorage.googleapis.com/dalecurtis-shared/alsa-vs-pulse.png

Which means I still need to look into writing some clock
smoothing in order for us to turn on renderer side mixing.

Unit tests to come later, mostly just getting it working now.

BUG=32757
TEST=media_unittests / manual playback tests.

Review URL: https://codereview.chromium.org/11098031

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@168116 0039d316-1c4b-4281-b951-d872f2087c98
6 files changed