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This will allow us to assess the impact of using thread priorities.
The impact can be both positive (unimportant work gets out of the way)
and negative (priority inversions when sharing locks across threads
with different priorities).

Note: This CL has extra complexity because we don't have guarantees
that the FeatureList is initialized before threads are started in
unit tests https://crbug.com/846380.

Bug: 872820, 890978, 902441, 846380
Change-Id: I8887c7b9e0eb77f7c11aa6e2be8af1ea7150b491
Reviewed-on: https://chromium-review.googlesource.com/c/1318686
Commit-Queue: François Doray <fdoray@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607276}