Instead of having a comfort noise generator (CNG) instance per capture
channel, one instance handles CNG for all capture channels.
Bug: webrtc:10913
Change-Id: I897471be6d203ad750c517c5076d421f2ae3879b
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/158780
Reviewed-by: Per Åhgren <peah@webrtc.org>
Commit-Queue: Gustaf Ullberg <gustaf@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29668}