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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.

CancelableTaskScheduler is no longer used and deleted in this CL.

Benchmark v8.browsing_desktop on win-10-perf browse:news:cnn:2018:
blink-gc-atomic-pause-mark-transitive-closure: 13% improvement
blink-gc-atomic-pause-sweep-and-compact: 5% improvement
blink-gc-complete-sweep: 34% improvement
blink-gc-total: 2% improvement
https://pinpoint-dot-chromeperf.appspot.com/job/136a60ce620000

Bug: 1046343
Change-Id: Ie2957abf200037ccacc017b1e38dc41f47811ee0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2029609
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#751502}