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This CL uses the lower bit of worker_count to indicate that
a JobTaskSource is canceled. It implements the logic
to cancel the task source and force workers to yield.

Bug: 839091
Change-Id: Ia621cd174d57dabdfa1df74e0b48a7afb20e8b51
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1772239
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#696907}