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Current implementation delegates to Join(), but there's no need to bump
priority of a canceled task. This CL simply calls WillJoin().

There's a subtle race that was hidden by priority bump
(which takes ThreadGroup::lock_).
1- Thread B calls GetRemainingConcurrency(). state_ is not canceled.
2- Thread A calls JobHandle::Cancel() and frees job state.
3- Thread B calls GetMaxConcurrency() -> use after free
RemoveJobTaskSource() becomes necessary again to make sure the
task_source doesn't become dangling while still in the queue.
Once JobHandle::Cancel() returns, it's ok if we end up with dangling
references of job_task_source since it's already canceled.

Change-Id: I6cf831b64fc75e555e32ae9b399342d64ed3f7d1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2369259
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#801497}