This CL adds the first layer of the echo canceller 3.
All of the code is as it should, apart from
block_processor.* which only contains placeholder
functionality. (Upcoming CLs will add proper
functionality into those files.)
BUG=webrtc:6018
Review-Url: https://codereview.webrtc.org/2584493002
Cr-Commit-Position: refs/heads/master@{#15861}