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This is a reland of fb56fbc5
Issue: Trace event under lock cause PostTask and lock inversion.
Fix: Move trace event outside of lock (to both callsites of
WaitForParticipationOpportunity), this
should give us mostly the same information but scope is a bit bigger.
See patchset 2

Original change's description:
> Reland "[Jobs API]: Use worker_lock in JobTaskSource."
>
> 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}

Change-Id: I0b08ee9fb7efd6b5cfdf9d171f2a280ccbb1fa5d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2309572Reviewed-by: Gabriel Charette <gab@chromium.org>
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#790489}