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This allows other processes to directly control the mixer, eg volume
control or postprocessor config changes.

Bug: internal b/127963522
Change-Id: Ibd2dc3b320ae633c9b7ddc9b51889bff34d7f62e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1842272Reviewed-by: Yuchen Liu <yucliu@chromium.org>
Commit-Queue: Kenneth MacKay <kmackay@chromium.org>
Cr-Commit-Position: refs/heads/master@{#704873}