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w/o this, WebAudio will always think there are only two channels
available. Untested since I don't have a 5.1 setup on Linux.

BUG=646435
TEST=verified that 2 channels are reported locally.

Review-Url: https://codereview.chromium.org/2345813002
Cr-Commit-Position: refs/heads/master@{#419052}