The number of output channels is constrained to be equal to either 1 or the number of input channels. An earlier version of this commit caused a crash on AEC dump. TBR=aluebs@webrtc.org,pbos@webrtc.org Review URL: https://codereview.webrtc.org/1248393003 . Cr-Commit-Position: refs/heads/master@{#9626}