Add ui::LayerObserver and use it to update Alt+Tab previews as needed.

ash/wm/window_mirror_view.h

Index: ash/wm/window_mirror_view.h
diff --git a/ash/wm/window_mirror_view.h b/ash/wm/window_mirror_view.h
index c91db5e09f11b7ef08cd363df57c8f31be5fdd41..1f9cd65416a4b0682670662d8eef4d735f626a97 100644
--- a/ash/wm/window_mirror_view.h
+++ b/ash/wm/window_mirror_view.h
@@ -10,6 +10,8 @@
 
 #include "ash/ash_export.h"
 #include "base/macros.h"
+#include "base/scoped_observer.h"
+#include "ui/compositor/layer_observer.h"
 #include "ui/views/view.h"
 #include "ui/wm/core/window_util.h"
 
@@ -24,7 +26,9 @@ class ForwardingLayerDelegate;
 // A view that mirrors the client area of a single window. Layers are lifted
 // from the underlying window (which gets new ones in their place). New paint
 // calls, if any, are forwarded to the underlying window.
-class WindowMirrorView : public views::View, public ::wm::LayerDelegateFactory {
+class WindowMirrorView : public views::View,
+                         public ::wm::LayerDelegateFactory,
+                         public ui::LayerObserver {
  public:
   explicit WindowMirrorView(WmWindowAura* window);
   ~WindowMirrorView() override;
@@ -38,6 +42,10 @@ class WindowMirrorView : public views::View, public ::wm::LayerDelegateFactory {
   // ::wm::LayerDelegateFactory:
   ui::LayerDelegate* CreateDelegate(ui::LayerDelegate* delegate) override;
 
+  // ui::LayerObserver:
+  void OnDelegatedFrameDamageInTree() override;
+  void OnLayerDestroyed(ui::Layer* layer) override;
+
  private:
   void InitLayerOwner();
 
@@ -56,6 +64,8 @@ class WindowMirrorView : public views::View, public ::wm::LayerDelegateFactory {
   // the first time the view becomes visible.
   std::unique_ptr<ui::LayerTreeOwner> layer_owner_;
 
+  ScopedObserver<ui::Layer, ui::LayerObserver> scoped_observer_;
+
   std::vector<std::unique_ptr<ForwardingLayerDelegate>> delegates_;
 
   DISALLOW_COPY_AND_ASSIGN(WindowMirrorView);