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This reverts commit 5d4c05db.

Reason for revert: Issue 950383

Original change's description:
> [TaskScheduler]: Simplify sequence preemption logic.
> 
> This CL removes preempted sequences from TaskTracker,
> removing need for CanScheduleSequenceObserver and
> simplifying shutdown.
> A CanRunPolicy enum is used instead, specifying whether
> or not tasks at a given priority can run.
> When CanRunPolicy is updated, wakeup workers needs
> to be done as appropriate.
> 
> In addition, Shutdown was split in 2 phases start + complete
> to allow caller (TaskScheduler) to update CanRunPolicy after
> shutdown was started.
> 
> This CL was recycled from:
> https://chromium-review.googlesource.com/c/chromium/src/+/1478007
> I took ownership because simplifying Sequence lifecycle
> will simplify implementation of job API.
> 
> TBR=gab@chromium.org
> 
> Bug: 889029
> Change-Id: Ie5dbc226705880c5e598e013aa2dd3e7e874158b
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1531161
> Reviewed-by: Etienne Pierre-Doray <etiennep@chromium.org>
> Reviewed-by: François Doray <fdoray@chromium.org>
> Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#648346}

TBR=gab@chromium.org,fdoray@chromium.org,etiennep@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 889029
Change-Id: Id15ef90caa4ddec43db925968268d2942a11fc1b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1557785Reviewed-by: Etienne Pierre-Doray <etiennep@chromium.org>
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#648673}