Reland "[TaskScheduler]: Simplify sequence preemption logic."

This is a reland of 5d4c05db6d02f9422b3a7b6fb2b6b637691ccc2e

Problem: PlatformNativeWorkerPool::RunNextSequenceImpl expects sequence
returned from GetWork() to be non-null and DCHECK.
Solution: Handle null sequences by skipping RunAndPopNextTask.

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}

Bug: 889029, 950383
Change-Id: I1f64517d312e65fb59ae235373b4f4d504da87dd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1557861
Commit-Queue: François Doray <fdoray@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#649399}