WebmMuxer: do not support >2 audio channels

ToT WebmMuxer supports Opus with >2 channels. This configuration is
never actually used (because AudioMediaStreams in Chrome don't
support it [1]) and can cause a write after bounds (see bug). This
CL removes the guilty code by not supporting >2 channels audio.

[1] https://chromium.googlesource.com/chromium/src/+/HEAD/third_party/blink/renderer/modules/mediarecorder/README.md#can-record-stereo

Bug: 930035
Change-Id: I964c66bc660e76ee152563804c63276643f4acd6
Reviewed-on: https://chromium-review.googlesource.com/c/1489185
Commit-Queue: Miguel Casas <mcasas@chromium.org>
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636056}
1 file changed