This CL contains the first step of adding multi-channel support to the echo subtractor. The CL is bitexact for the mono case. Bug: webrtc:10913 Change-Id: I10647b45c692bc001407afc6ff00e26a3e2cffaa Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/154356 Reviewed-by: Gustaf Ullberg <gustaf@webrtc.org> Commit-Queue: Per Åhgren <peah@webrtc.org> Cr-Commit-Position: refs/heads/master@{#29303}