tablet: Speculative fix for signature ui::LayerAnimator::RemoveObserver.

Stack trace shows window destroying calling |animating_layer_| which is
the one of the windows associated with the destroyed window. My guess is
that it might be a dangling pointer, so when window is destroying, set
it to null.

I tried reproing with a test that destroys the windows at certain points
of the animation, but no luck :(.

Test: none
Bug: 1028815
Change-Id: I5baa2d3784836296c2afe063e6861aa43fb68e5f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2111059
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#752670}

ash/wm/tablet_mode/tablet_mode_controller.cc

@@ -426,6 +426,7 @@ void TabletModeController::MaybeObserveBoundsAnimation(aura::Window* window) {
                              /*delete_screenshot=*/true));
   animating_layer_ = window->layer();
   animating_layer_->GetAnimator()->AddObserver(this);
+  animating_layer_->AddObserver(this);
 }
 
 void TabletModeController::StopObservingAnimation(bool record_stats,
@@ -434,9 +435,17 @@ void TabletModeController::StopObservingAnimation(bool record_stats,
 
   ResetDestroyObserver();
 
-  if (animating_layer_)
-    animating_layer_->GetAnimator()->RemoveObserver(this);
-  animating_layer_ = nullptr;
+  if (animating_layer_) {
+    animating_layer_->GetAnimator()->StopAnimating();
+
+    // If the observed layer is part of a cross fade animation, stopping the
+    // animation will end up destroying the layer.
+    if (animating_layer_) {
+      animating_layer_->RemoveObserver(this);
+      animating_layer_->GetAnimator()->RemoveObserver(this);
+      animating_layer_ = nullptr;
+    }
+  }
 
   if (record_stats && fps_counter_)
     fps_counter_->LogUma();
@@ -695,6 +704,13 @@ void TabletModeController::OnLayerAnimationScheduled(
   StopObservingAnimation(/*record_stats=*/false, /*delete_screenshot=*/true);
 }
 
+void TabletModeController::LayerDestroyed(ui::Layer* layer) {
+  DCHECK_EQ(animating_layer_, layer);
+  animating_layer_->RemoveObserver(this);
+  animating_layer_->GetAnimator()->RemoveObserver(this);
+  animating_layer_ = nullptr;
+}
+
 void TabletModeController::SetEnabledForDev(bool enabled) {
   tablet_mode_behavior_ = enabled ? kOnForDev : kDefault;
   force_notify_events_blocking_changed_ = true;

ash/wm/tablet_mode/tablet_mode_controller.h

@@ -25,6 +25,7 @@
 #include "chromeos/dbus/power/power_manager_client.h"
 #include "ui/aura/window_occlusion_tracker.h"
 #include "ui/compositor/layer_animation_observer.h"
+#include "ui/compositor/layer_observer.h"
 #include "ui/compositor/layer_tree_owner.h"
 #include "ui/events/devices/input_device_event_observer.h"
 #include "ui/gfx/geometry/vector3d_f.h"
@@ -71,7 +72,8 @@ class ASH_EXPORT TabletModeController
       public WindowTreeHostManager::Observer,
       public SessionObserver,
       public ui::InputDeviceEventObserver,
-      public ui::LayerAnimationObserver {
+      public ui::LayerAnimationObserver,
+      public ui::LayerObserver {
  public:
   // Enable or disable using a screenshot for testing as it makes the
   // initialization flow async, which makes most tests harder to write.
@@ -158,6 +160,9 @@ class ASH_EXPORT TabletModeController
   void OnLayerAnimationAborted(ui::LayerAnimationSequence* sequence) override;
   void OnLayerAnimationScheduled(ui::LayerAnimationSequence* sequence) override;
 
+  // ui::LayerObserver:
+  void LayerDestroyed(ui::Layer* layer) override;
+
   void increment_app_window_drag_count() { ++app_window_drag_count_; }
   void increment_app_window_drag_in_splitview_count() {
     ++app_window_drag_in_splitview_count_;
@@ -447,6 +452,8 @@ class ASH_EXPORT TabletModeController
   // everything in the screen rotation container except the top window. It helps
   // with animation performance because it fully occludes all windows except the
   // animating window for the duration of the animation.
+  // TODO(sammiequon): See if we can move screenshot and tablet mode transition
+  // animation related code into a separate class/file.
   std::unique_ptr<ui::Layer> screenshot_layer_;
 
   base::ObserverList<TabletModeObserver>::Unchecked tablet_mode_observers_;