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The new backend reuses most of ALSA backend. There are two parts that
are different:
 - Added new AudioOutputStreamFuchsia to be used instead of
   AudioOutputStreamAlsa.
 - Added stub for SystemVolumeControl.

Bug: 772488
Change-Id: Icacb070323f30c3d7e6a50bd1f5cf5a58c4251f9
Reviewed-on: https://chromium-review.googlesource.com/722323
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Reviewed-by: James Robinson <jamesr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#510116}