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Manual reland:
Original CL: https://chromium-review.googlesource.com/c/chromium/src/+/2029609
Revert: https://chromium-review.googlesource.com/c/chromium/src/+/2109970
Reset for revert: flaky DCHECK in AssertExpectedConcurrency
Fix: Avoid calling AssertExpectedConcurrency from ShouldYield.
https://chromium-review.googlesource.com/c/chromium/src/+/2112235

Original Description:
Note: This CL enables feature BlinkHeapConcurrentMarking to get
the desired behavior. However, we should land enabling
BlinkHeapConcurrentMarking on its own first.

active_markers + GlobalPoolSize() is used to determine
the desired number of workers.

NotifyConcurrencyIncrease() is called periodically from
ConcurrentMarkingStep() if GlobalPoolSize() > 0,
to make sure enough workers are scheduled.

Bug: 1046343
Change-Id: I4c36044686128b23fc25f45b99334c219a08cffa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2214908Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#779969}