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It's not clear to me why calling base::ThreadTaskRunnerHandle::Get()
from ArcProcessTaskProvider::ScheduleNextRequest() would fail,
especially since this is happening on the UI thread. But I added a few
things that may fix the issue:
- I noticed that ArcProcessService dtor may be called, but we did't
  stop observing the ProcessSnapshotServer.
- When the task manager is closed, let's invalidate all the weak_ptrs of
  ArcProcessTaskProvider, so we don't get any pending requested from
  ArcProcessService after that.

BUG=1136378

Change-Id: Icdbc8161e397899a2decd7efac13a39462bb6d71
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2536812Reviewed-by: Willie Koomson <wvk@google.com>
Commit-Queue: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827898}