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Issue: Idle thread creation is not disallowed while JoinForTesting().
Fix: prevent thread creation in EnsureEnoughWorkersLockRequired with join_for_testing_started_.

Bug: 1009536
Change-Id: I3f6f18efa5e950c69fef1f7d4458d703ca1d6056
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1834201
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#703347}