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CAOS was refactored to simplify the logic for the service connector,
multithreading, and unit tests.

The CAOS::Backend object was renamed to CAOS::CmaWrapper, and much of
the implementation was moved out into CAOS to simplify the object.

The service connector is created in CastAudioManager and passed to
CAOS's audio thread. Instead of passing this then to the media thread
as done previously, we only utilize the connector in the audio thread.
The connector can be easily overriden for tests.

CAOS runs on two threads. It is constructed and controlled from
CastAudioManager in the audio thread. All CMA construction and control
happens in the media thread in the CAOS::CmaWrapper object.

The thread usage in the unit tests has been significantly simplified.
Instead of using RunLoops and multiple threads, we use a single thread
retrieved from a ScopedTaskEnvironment in CAM, CAOS:audio, and CAOS:media.
This means that everything in the unit tests are running on a single
thread, and the tests are far less flaky.

In order to allow everything to run on a single thread, the usages of
WaitableEvent were removed. WaitableEvent was previously used to allow
the synchronous behavior of the audio thread to work with the
asynchronous thread hopping. To compensate for this, certain task
postings between threads can be cached or cancelled, and errors are
returned late.

The following unit tests were added to verify the above behavior and
fill in untested implementation:
- CastAudioManagerTest
  .HasValidOutputStreamParameters
  .CanMakeStream
  .CanMakeMixerStream
- CastAudioOutputStreamTest
  .CloseWithoutStart
  .CloseWithoutStop
  .CloseCancelsOpen
  .CloseCancelsStart
  .CloseCancelsStop
  .StartImmediatelyAfterOpen
  .SetVolumeImmediatelyAfterOpen
  .StopCancelsStart
  .StopDoesNotCancelSetVolume
  .PushFrameAfterStop
  .PushFrameAfterClose
  .MultiroomInfo
  .SessionId

Bug: b/111993375
Test: cast_media_unittests, udon build
Change-Id: Ib7dce8df8a36a2311e13d6a0062b7e56d4928995
Reviewed-on: https://chromium-review.googlesource.com/1159824
Commit-Queue: Aidan Wolter <awolter@chromium.org>
Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Reviewed-by: Kenneth MacKay <kmackay@chromium.org>
Cr-Commit-Position: refs/heads/master@{#586096}