[Chromecast] Allow arbitrary channel count for audio streams

Bug: internal b/138305562
Test: cast_audio_backend_unittests
Change-Id: I6435648a0ca13656975217aa389682ea5ace6bc6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1752925
Commit-Queue: Kenneth MacKay <kmackay@chromium.org>
Reviewed-by: Yuchen Liu <yucliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#686935}
40 files changed