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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}