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This CL wires up the experiment settings for tuning WebRTC's gain
control. Four different new values are provided as feature flags (to
be controlled by a Finch experiment or directly from command line
flags).

BUG=chromium:672476,webrtc:6622

Review-Url: https://codereview.chromium.org/2560173002
Cr-Commit-Position: refs/heads/master@{#437847}