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This CL provides an experimental AudioCapturer implementation for Mac.
It's only enabled in debug builds. Since Mac doesn't provide an API to
capture system-wide audio, the way it works is to have the user install
some kind of loopback CoreAudio plugin like BlackHole, then make it both
the input and output device. The user will also need to go to the clunky
"Security & Privacy" panel and give Microphone permission to our app.

This setup streams audio quite well and I didn't notice any glitches,
though you'll need to have everything set up correctly in order to use
this experimental capturer.

Bug: 1161363
Change-Id: Id79a9bc2d9a7cdc788b3f0405432b99a31d08930
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2601551
Commit-Queue: Yuwei Huang <yuweih@chromium.org>
Reviewed-by: Joe Downing <joedow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#840393}