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This CL Creates a new derived JobTaskSource that tracks the number
of concurrent worker.
It's a precursor of https://chromium-review.googlesource.com/c/chromium/src/+/1582427
which integrates JobTaskSource in ThreadGroups.

Bug: 839091
Change-Id: Ie407823bdd6d4c84fd8b3db7d6d6a58a33846045
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1693182
Commit-Queue: François Doray <fdoray@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#677058}