This CL adds basic the basic pipeline to support multi-channel processing in AEC3. Apart from that, it removes the 8 kHz processing support in several places of the AEC3 code. Bug: webrtc:10913 Change-Id: If5b75fa325ed0071deea94a7546cb4a7adf22137 Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/150332 Commit-Queue: Per Åhgren <peah@webrtc.org> Reviewed-by: Sam Zackrisson <saza@webrtc.org> Cr-Commit-Position: refs/heads/master@{#29017}