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This CL is fixing a race condition that can happen when two threads
are trying to load a DLL at the same time. The problem was that the
priority boost may happen on the wrong thread
(see: https://crbug.com/1124759).

The solution is to add a std::atomic_bool already_loaded and set the
flag when exiting the scope. It is fine that two threads enter the
scope and try to load the DLL at the same time. Both thread will have
and increased thread priority.

R=gab@chromium.org, fdoray@chromium.org

Bug: 1124759
Change-Id: I650ff99f56f1abeba6175024909cf344cad6bc61
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2392746Reviewed-by: François Doray <fdoray@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Commit-Queue: Etienne Bergeron <etienneb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805083}