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This is a reland of https://crrev.com/c/chromium/src/+/1865405
The difference is that we don't check thread priority in
angle_unittest, as these tests change the thread priority on
purpose.

This CL verifies that the thread priority is normal when a test process
is launched and before/after each test. The goal is to avoid having
tests that assume they run at normal priority be disabled because of
other misbehaving tests (e.g. https://crbug.com/931706).

TBR=sky@chromium.org,michaelpg@chromium.org,thakis@chromium.org,eseckler@chromium.org

Bug: 931706
Change-Id: I8fb2230cbb9b9203884d1c440696d4ce2968dcaf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1881707Reviewed-by: Michael Giuffrida <michaelpg@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710326}