Skip clearing the canvas before painting the contents of opaque layers

cc/resources/content_layer_updater.h

 // Copyright 2011 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_RESOURCES_CONTENT_LAYER_UPDATER_H_
 #define CC_RESOURCES_CONTENT_LAYER_UPDATER_H_
 
 #include "cc/base/cc_export.h"
 #include "cc/resources/layer_updater.h"
 #include "ui/gfx/rect.h"
 
 class SkCanvas;
 
 namespace cc {
 
 class LayerPainter;
 class RenderingStatsInstrumentation;
 
 // Base class for BitmapContentLayerUpdater and
 // SkPictureContentLayerUpdater that reduces code duplication between
 // their respective PaintContents implementations.
 class CC_EXPORT ContentLayerUpdater : public LayerUpdater {
  public:
   void set_rendering_stats_instrumentation(RenderingStatsInstrumentation* rsi);
+  virtual void SetOpaque(bool) OVERRIDE;
 
  protected:
   ContentLayerUpdater(scoped_ptr<LayerPainter> painter,
                       RenderingStatsInstrumentation* stats_instrumentation,
                       int layer_id);
   virtual ~ContentLayerUpdater();
 
   void PaintContents(SkCanvas* canvas,
-                     gfx::Rect content_rect,
+                     gfx::Point origin,
                      float contents_width_scale,
                      float contents_height_scale,
                      gfx::Rect* resulting_opaque_rect);
   gfx::Rect content_rect() const { return content_rect_; }
 
+  bool layer_is_opaque() const { return layer_is_opaque_; }
+
   RenderingStatsInstrumentation* rendering_stats_instrumentation_;
   int layer_id_;
 
  private:
   gfx::Rect content_rect_;
   scoped_ptr<LayerPainter> painter_;
 
+ protected:
+  // True when it is known that all output pixels will be opaque.
+  bool layer_is_opaque_;
+
   DISALLOW_COPY_AND_ASSIGN(ContentLayerUpdater);
 };
 
 }  // namespace cc
 
 #endif  // CC_RESOURCES_CONTENT_LAYER_UPDATER_H_