fix heap-use-after-free when aborting an animation

Under certain circumstances, such as aborting an animation and then
reassigning a unique_ptr to a new CallbackLayerAnimationObserver (eg
InkDropRipple::SnapToState(InkDropState ink_drop_state), the logic to
detect deletion in CallbackLayerAnimationObserver ends up causing a
heap-use-after-free because
CallbackLayerAnimationObserver::CheckAllSequencesCompleted() overwrites
the destroyed_ pointer that CallbackLayerAnimationObserver::SetActive() set.

In order to fix this issue, use of the destroyed_ pointer was replaced
with local WeakPtrs in order to detect deletion.

Change-Id: I051657fae929eb4d7200f965561e9d9fb21b44f5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1714626
Reviewed-by: kylechar <kylechar@chromium.org>
Commit-Queue: Jeremy Apthorp <jeremya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#683029}

• AUTHORS[diff]
• ui/compositor/callback_layer_animation_observer.cc[diff]
• ui/compositor/callback_layer_animation_observer.h[diff]
• ui/compositor/callback_layer_animation_observer_unittest.cc[diff]