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There are some concerns about increased audio rendering glitches in Beta finch experiment (go/mac-rt-thread-finch).
They may be caused either by the renderer side RT audio thread being descheduled since it's made "preemptible",
or by multiple RT audio threads competing for CPU since we relaxed their deadlines.
This CL is exposing the controls to allow for further experimentation around that as needed.

The plan is to backmerge this to M88 and to run an
 3-leg M88 experiment with "preemptible" set to false and true in experiment groups..
Meanwhile, there is 1% M87 stable experiment to confirm positive effects on Meet we've observed in M87 Beta.

Bug: 1132225
Change-Id: I7a79f5a0317814ff16252e0d88db8d61d5af599c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2546712
Commit-Queue: Olga Sharonova <olka@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Auto-Submit: Olga Sharonova <olka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#829231}