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Conditions:
* Track processing is enabled.
* Experimental constraint is set.
* Keyboard mic is supported (reported via effect).
* The ordinary layout is stereo.

This CL will be landed last of the keyboard mic support CLs.

BUG=345296

Review URL: https://codereview.chromium.org/598563002

Cr-Commit-Position: refs/heads/master@{#296689}