Add testing config for MainThreadCompositingPriority on ChromeOS.

A previous experiment with this feature confirmed Core Web Vitals
improvements, but had an audio glitches regressions. We believe that
this regression is no longer present with crrev.com/c/4647228. This
experiment will be used to confirm that.

Note: MainThreadCompositingPriority is ENABLED_BY_DEFAULT so this
CL should be a no-op. Nonetheless, it is required to run our
experiment.

Bug: 1406005
Change-Id: I003b6381eadf0a7b87e9972c96c5442e7abf6705
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4791161


Reviewed-by: Etienne Pierre-Doray <etiennep@chromium.org>
Auto-Submit: Francois Pierre Doray <fdoray@chromium.org>
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1184825}