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queue_time for Jobs is adjusted based on number of workers to put
back pressure on Jobs that use many workers. This prioritizes newer work
that uses less workers (or simple sequences).
To achieve this, GetSortKey is extracted from Transaction and uses
atomic load. This is safe since the result of GetSortKey and the queue
itself is only modified under lock.

Follow-up: consider integrating full TaskSourceSortKey in ShouldYield

Change-Id: If26e00b40502b0a4e09f10e3627de8b7e4a52488
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2377893
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#807090}