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

ui/compositor/layer.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef UI_COMPOSITOR_LAYER_H_
 #define UI_COMPOSITOR_LAYER_H_
 
 #include <stddef.h>
 
 #include <memory>
 #include <string>
 #include <vector>
 
 #include "base/compiler_specific.h"
 #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"
 #include "cc/layers/surface_layer.h"
 #include "cc/layers/texture_layer_client.h"
 #include "cc/resources/texture_mailbox.h"
 #include "cc/surfaces/surface_id.h"
 #include "third_party/skia/include/core/SkColor.h"
 #include "third_party/skia/include/core/SkRegion.h"
 #include "ui/compositor/compositor.h"
@@ skipping 16 matching lines @@
 class SurfaceLayer;
 class TextureLayer;
 struct ReturnedResource;
 typedef std::vector<ReturnedResource> ReturnedResourceArray;
 }
 
 namespace ui {
 
 class Compositor;
 class LayerAnimator;
+class LayerObserver;
 class LayerOwner;
 class LayerThreadedAnimationDelegate;
 
 // Layer manages a texture, transform and a set of child Layers. Any View that
 // has enabled layers ends up creating a Layer to manage the texture.
 // A Layer can also be created without a texture, in which case it renders
 // nothing and is simply used as a node in a hierarchy of layers.
 // Coordinate system used in layers is DIP (Density Independent Pixel)
 // coordinates unless explicitly mentioned as pixel coordinates.
 //
@@ skipping 22 matching lines @@
   // only ever be called on the root layer.
   void SetCompositor(Compositor* compositor,
                      scoped_refptr<cc::Layer> root_layer);
   void ResetCompositor();
 
   LayerDelegate* delegate() { return delegate_; }
   void set_delegate(LayerDelegate* delegate) { delegate_ = delegate; }
 
   LayerOwner* owner() { return owner_; }
 
+  void AddObserver(LayerObserver* observer);
+  void RemoveObserver(LayerObserver* observer);
+
   // Adds a new Layer to this Layer.
   void Add(Layer* child);
 
   // Removes a Layer from this Layer.
   void Remove(Layer* child);
 
   // Stacks |child| above all other children.
   void StackAtTop(Layer* child);
 
   // Stacks |child| directly above |other|.  Both must be children of this
@@ skipping 389 matching lines @@
   // Width of the border in pixels, where the scaling is blended.
   int zoom_inset_;
 
   // Shape of the window.
   std::unique_ptr<SkRegion> alpha_shape_;
 
   std::string name_;
 
   LayerDelegate* delegate_;
 
+  base::ObserverList<LayerObserver> observer_list_;
+
   LayerOwner* owner_;
 
   scoped_refptr<LayerAnimator> animator_;
 
   // Ownership of the layer is held through one of the strongly typed layer
   // pointers, depending on which sort of layer this is.
   scoped_refptr<cc::Layer> content_layer_;
   scoped_refptr<cc::NinePatchLayer> nine_patch_layer_;
   scoped_refptr<cc::TextureLayer> texture_layer_;
   scoped_refptr<cc::SolidColorLayer> solid_color_layer_;
@@ skipping 25 matching lines @@
   // The texture scale.
   float texture_x_scale_;
   float texture_y_scale_;
 
   DISALLOW_COPY_AND_ASSIGN(Layer);
 };
 
 }  // namespace ui
 
 #endif  // UI_COMPOSITOR_LAYER_H_