Allow more than 2 input channels in AudioProcessing.

The number of output channels is constrained to be equal to either 1 or the
number of input channels.

R=aluebs@webrtc.org, andrew@webrtc.org, pbos@webrtc.org

Review URL: https://codereview.webrtc.org/1226093007 .

Cr-Commit-Position: refs/heads/master@{#9619}
13 files changed