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

ui/compositor/layer.h

Index: ui/compositor/layer.h
diff --git a/ui/compositor/layer.h b/ui/compositor/layer.h
index dc28e09259ed347220dffb0ec2fa61927313c0c6..f892fef3552c753aede021d9431119f716b63362 100644
--- a/ui/compositor/layer.h
+++ b/ui/compositor/layer.h
@@ -15,6 +15,7 @@
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/message_loop/message_loop.h"
+#include "base/observer_list.h"
 #include "cc/base/region.h"
 #include "cc/layers/content_layer_client.h"
 #include "cc/layers/layer_client.h"
@@ -51,6 +52,7 @@ namespace ui {
 
 class Compositor;
 class LayerAnimator;
+class LayerObserver;
 class LayerOwner;
 class LayerThreadedAnimationDelegate;
 
@@ -93,6 +95,9 @@ class COMPOSITOR_EXPORT Layer
 
   LayerOwner* owner() { return owner_; }
 
+  void AddObserver(LayerObserver* observer);
+  void RemoveObserver(LayerObserver* observer);
+
   // Adds a new Layer to this Layer.
   void Add(Layer* child);
 
@@ -503,6 +508,8 @@ class COMPOSITOR_EXPORT Layer
 
   LayerDelegate* delegate_;
 
+  base::ObserverList<LayerObserver> observer_list_;
+
   LayerOwner* owner_;
 
   scoped_refptr<LayerAnimator> animator_;