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1. Fix the following problem: When Stopped,
CastAudioOutputStream would crash stopping the
|io_thread_|, which is also unecessary.

2. Make the unit tests of CastAudioOutputStream
and CastAudioManager only test CMABackend implementation
of CastAudioOutputStream because they are not ready to
test the newer MixerService implementation.

Bug: internal b/29571387
Test:
1. Play a lasting sin wave audio on two devices, to make
sure the AudioOutputStream plays fine whether or not MixerService
is used.
2. Run cast_media_unittests on estelle and all the tests passed.
3. Run internal CQ and the CQ including cast_media_unittests passed.

Change-Id: I380991235344348af56293ae214fb8c0e2be80b0
Reviewed-on: https://chromium-review.googlesource.com/c/1294190
Commit-Queue: Guohui Deng <guohuideng@chromium.org>
Reviewed-by: Kenneth MacKay <kmackay@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601818}