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This is a reland of 6eb566d2

Reason for revert: Cause failure in CheckedLockImpl::Acquire
crbug.com/1099649
Mark worker_released_condition_ declare_only_used_while_idle
to prevent priority queue from being acquired in ScopedBlockingCall.

Original change's description:
> [Jobs API]: Use worker_lock in JobTaskSource.
>
> Possible race when Join():
> - thread A: Join: worker_released_condition_->Wait()
> - thread C: WillRunTask:
>               GetMaxConcurrency() returns > 0
> - thread B: already running, finishes all the work
>               GetMaxConcurrency() goes 0
> - thread B: DidProcessTask:
>               worker_released_condition_->Signal(),
> - thread A: Join returns (GetMaxConcurrency() is 0)
> - thread C: TryIncrementWorkerCountFromWorkerRelease
>               worker count goes 1
> - thread C: runs worker_task after Join
>
> To fix race when Joining, all writes to |state_| are protected by
> |worker_lock|. Memory ordering is no longer necessary.
>
> Alternative: cancel before Join returns with a compare and swap and
> loop again if new workers.
>
> Change-Id: I4e478ffae2bdaec56386739f78de089d0e74e42c
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2248159
> Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
> Reviewed-by: Gabriel Charette <gab@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#781453}

Change-Id: I1c7c0054a52b9b12dd6d0edd049ab2a7912df361
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2272942
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#789526}