Skip clearing the canvas before painting the contents of opaque layers

cc/resources/skpicture_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_SKPICTURE_CONTENT_LAYER_UPDATER_H_
 #define CC_RESOURCES_SKPICTURE_CONTENT_LAYER_UPDATER_H_
 
 #include "cc/resources/content_layer_updater.h"
 #include "third_party/skia/include/core/SkPicture.h"
 
 class SkCanvas;
 
 namespace cc {
 
 class LayerPainter;
 
 // This class records the content_rect into an SkPicture. Subclasses, provide
 // different implementations of tile updating based on this recorded picture.
 // The BitmapSkPictureContentLayerUpdater and
 // FrameBufferSkPictureContentLayerUpdater are two examples of such
 // implementations.
 class SkPictureContentLayerUpdater : public ContentLayerUpdater {
- public:
-  virtual void SetOpaque(bool opaque) OVERRIDE;
-
  protected:
   SkPictureContentLayerUpdater(
       scoped_ptr<LayerPainter> painter,
       RenderingStatsInstrumentation* stats_instrumentation,
       int layer_id);
   virtual ~SkPictureContentLayerUpdater();
 
   virtual void PrepareToUpdate(gfx::Rect content_rect,
                                gfx::Size tile_size,
                                float contents_width_scale,
                                float contents_height_scale,
                                gfx::Rect* resulting_opaque_rect) OVERRIDE;
   void DrawPicture(SkCanvas* canvas);
 
-  bool layer_is_opaque() const { return layer_is_opaque_; }
-
  private:
   // Recording canvas.
   SkPicture picture_;
-  // True when it is known that all output pixels will be opaque.
-  bool layer_is_opaque_;
 
   DISALLOW_COPY_AND_ASSIGN(SkPictureContentLayerUpdater);
 };
 
 }  // namespace cc
 
 #endif  // CC_RESOURCES_SKPICTURE_CONTENT_LAYER_UPDATER_H_