Keeping track of descendants that draw content instead of recalcualting

cc/layers/image_layer.h

 // Copyright 2010 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 CC_LAYERS_IMAGE_LAYER_H_
 #define CC_LAYERS_IMAGE_LAYER_H_
 
 #include "cc/base/cc_export.h"
 #include "cc/layers/content_layer.h"
 #include "third_party/skia/include/core/SkBitmap.h"
 
 namespace cc {
 
 class ImageLayerUpdater;
 
 // A Layer that contains only an Image element.
 class CC_EXPORT ImageLayer : public TiledLayer {
  public:
   static scoped_refptr<ImageLayer> Create();
 
   // Layer implementation.
-  virtual bool DrawsContent() const OVERRIDE;
   virtual void SetTexturePriorities(const PriorityCalculator& priority_calc)
       OVERRIDE;
   virtual bool Update(ResourceUpdateQueue* queue,
                       const OcclusionTracker<Layer>* occlusion) OVERRIDE;
   virtual void CalculateContentsScale(float ideal_contents_scale,
                                       float device_scale_factor,
                                       float page_scale_factor,
                                       float maximum_animation_contents_scale,
                                       bool animating_transform_to_screen,
                                       float* contents_scale_x,
                                       float* contents_scale_y,
                                       gfx::Size* content_bounds) OVERRIDE;
   virtual void OnOutputSurfaceCreated() OVERRIDE;
 
   void SetBitmap(const SkBitmap& image);
 
+ protected:
+  virtual bool HasDrawableContent() const OVERRIDE;
+
  private:
   ImageLayer();
   virtual ~ImageLayer();
 
   // TiledLayer Implementation.
   virtual LayerUpdater* Updater() const OVERRIDE;
   virtual void CreateUpdaterIfNeeded() OVERRIDE;
 
   float ImageContentsScaleX() const;
   float ImageContentsScaleY() const;
 
   SkBitmap bitmap_;
 
   scoped_refptr<ImageLayerUpdater> updater_;
 
   DISALLOW_COPY_AND_ASSIGN(ImageLayer);
 };
 
 }  // namespace cc
 
 #endif  // CC_LAYERS_IMAGE_LAYER_H_