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This is the complimentary testing docs to the Threading and Tasks usage
docs.

Updated in-code comments to reflect the current reality better too.

R=fdoray@chromium.org

Bug: 1002654
Change-Id: Ic9f4227e462ddc7180a1a6f3005935f1203ff064
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1798985
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: Etienne Pierre-Doray <etiennep@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#697592}