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This change is required to let Chrome encode and decode multistream
Opus audio. It adds WebRTC multistream Opus decoders and encoders as
non-advertised codecs to the audio codec factory.

The WebRTC default audio codec factory has contained the multistream
opus encoder/decoder since April 25 and April 8 respectively. The
multistream encoders have been tested in WebRTC.

Bug: webrtc:8649
Change-Id: Ie350ab2e052edfd7f18dfa4c67bb397dfa541d1d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1609782Reviewed-by: Henrik Boström <hbos@chromium.org>
Commit-Queue: Alex Loiko <aleloi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659048}