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This CL introduces MediaStreamAudioTrackUnderlyingSource. It is an
audio sink (in the MediaStream sense) which receives audio data. It
is also an underlying source (in the Streams API sense), which converts
the data received from an audio track into blink::AudioFrames, which
can then be pulled from a ReadableStream.

The CL also removes PushableAudioData and replaces it with
AudioFrameSerializationData, which will be improved upon (or replaced)
in crbug.com/1168418.

Bug: 1157608, 1168418
Change-Id: I8230ada4331899d8f1ef7373fb3f3eeb9a5fde95
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2640913
Commit-Queue: Thomas Guilbert <tguilbert@chromium.org>
Auto-Submit: Thomas Guilbert <tguilbert@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Reviewed-by: Chrome Cunningham <chcunningham@chromium.org>
Cr-Commit-Position: refs/heads/master@{#846314}