Stop observing all animation sequences before deleting

StackedNotificationBarIcon.

Issue 100% reproducible without the fix.
With the fix the crash does not happen.
Confirmed over 3 runs for each case.

Bug: 1050685
Change-Id: Id3e5f7204f7f124a1d258b2f45864b451ff6647a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2055846Reviewed-by: Tim Song <tengs@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Ahmed Mehfooz <amehfooz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#741586}

ash/system/message_center/stacked_notification_bar.cc

@@ -118,6 +118,7 @@ class StackedNotificationBar::StackedNotificationBarIcon
   }
 
   ~StackedNotificationBarIcon() override {
+    StopObserving();
     if (is_animating_out())
       layer()->GetAnimator()->StopAnimating();
   }