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This CL sampling the current process priority and add an instant event
to report it.

This will be used to figure out some priority inversion related to the
GPU process and background renderers.

R=oysteine@chromium.org

Change-Id: I9e2f9b4b48379ade4d476dd7c7fd2b2fe52639d7
Reviewed-on: https://chromium-review.googlesource.com/c/1350189Reviewed-by: oysteine <oysteine@chromium.org>
Commit-Queue: Etienne Bergeron <etienneb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611393}