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This CL adds a new MediaPipelineBackend to allow audio data to be
proxied across an IPC through the MultizoneAudioDecoderProxy. Video data
decoding will not be changed, and should be passed through to the
environment-specific CmaBackend. This change is accomplished through the
use of an environment variable orthogonal to that used for platform type.

After this change, all audio data processed by the AudioPipelineImpl
should be diverted across the IPC, as described in
go/cast-media-chromium-runtime

Bug: b/167425332
Change-Id: Iaefd150bac0b6edf9708c79d4c6d50f2a5eb27ae
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2412558Reviewed-by: Yuchen Liu <yucliu@chromium.org>
Commit-Queue: Ryan Keane <rwkeane@google.com>
Cr-Commit-Position: refs/heads/master@{#809635}