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This is a reland of b7526e4f, but
by with an additional fix.

When a PerProfileWorkerTaskTracker instance is deleted because
StopUpdating() was called, then the removal notifications must
not be sent to the task manager (as part of the TaskProvider's
API contract).

This is handled in WorkerTaskProvider by dropping the notification
when IsUpdating() is false.

Original change's description:
> [Task Manager] Properly clear worker tasks on profile shutdown.
>
> Fixes PerProfileWorkerTaskTracker so that the WorkerTaskTracker is
> notified when there are outstanding tasks that are about to be
> deleted.
>
> Bug: 1060506
> Change-Id: Ic81ad1557c1e27a8fc96fc079f7447efcc2ce7f3
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2110350
> Auto-Submit: Patrick Monette <pmonette@chromium.org>
> Commit-Queue: Ahmed Fakhry <afakhry@chromium.org>
> Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#751915}

Bug: 1060506
Change-Id: I3c1a7f8f94a580d021b7e3658af885110bfa8d2b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2118770
Commit-Queue: Patrick Monette <pmonette@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#753519}