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Pipe the number of encoded outgoing channels to the media stream audio
processor and use it to configure the Audio Processing Module (APM).
Stereo calls gets audio processing (echo cancellation, noise
suppression, etc) in stereo. Mono calls will continue to get mono
audio processing.

Previous behavior was to perform audio processing in mono and upmix
to two (identical) outgoing channels.

Bug: webrtc:8133
Change-Id: I97bfa8f6b9736a6e8e0d216c1795d712f075726e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2593122
Commit-Queue: Gustaf Ullberg <gustaf@chromium.org>
Reviewed-by: Olga Sharonova <olka@chromium.org>
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Reviewed-by: Henrik Andreasson <henrika@chromium.org>
Cr-Commit-Position: refs/heads/master@{#842577}