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base::CancelableTaskTracker is sequence-affine. Since it is not
possible to ensure that -dealloc is called at a specific point
in time nor on a specific thread for an Objective-C instance,
change _largeIconTaskTracker to be heap allocated and stored in
a std::unique_ptr<> so that it can be destroyed from -shutdown.

Add DCHECK that method accessing _largeIconTaskTracker are not
called after -shutdown as been called (as they would otherwise
try to dereference null which is Undefined Behaviour).

Convert _pendingTasks to a std::set<URL> since the value are
never used, and check that the scheduling of the task was a
success before storing the object in _pendingTasks (failure
probably mean that the sequence has been shutdown which only
happen at app shutdown, so it is okay to do nothing in that
case).

Bug: 1122991
Change-Id: I9f312bf6b8ac844e6a28d4e10042e9b9c1a7b529
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2388061
Commit-Queue: Olivier Robin <olivierrobin@chromium.org>
Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#804952}