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}