AECM only supports up to two capture channels, this CL extends it to arbitrary channel counts. Bug: webrtc:10859 Change-Id: Id56ca633cd9de706fa1254bfa8153de88de0ef70 Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/160340 Reviewed-by: Per Åhgren <peah@webrtc.org> Commit-Queue: Sam Zackrisson <saza@webrtc.org> Cr-Commit-Position: refs/heads/master@{#29880}