Replace num_proc_channels() with num_output_channels() in
GainController2. The number of channels is only used in
InputVolumeController.
Bug: webrtc:7494
Change-Id: I6b3f66980a518401fefab304e18c9910eee28d4e
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/338922
Reviewed-by: Per Åhgren <peah@webrtc.org>
Commit-Queue: Hanna Silen <silen@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#41717}
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