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This CL partially implement job's delegate, as well as concurrency
usage assertion:
- ShouldYield must be called in worker task.
- max concurrency is expected to decrease unless
  NotifyConcurrencyIncrease() is called.

To implement ShouldYield, PooledTaskRunnerDelegate is used
to communicate with ThreadPool and a racy priority is added to
task source.

Bug: 839091
Change-Id: I3504f00ec48ab600f79b64e44151bc9dded408a8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1713146
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#690127}