Dirty rects always contain full tiles with delegated rendering.

cc/resources/caching_bitmap_content_layer_updater.h

 // Copyright 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 CC_RESOURCES_CACHING_BITMAP_CONTENT_LAYER_UPDATER_H_
 #define CC_RESOURCES_CACHING_BITMAP_CONTENT_LAYER_UPDATER_H_
 
 #include "base/compiler_specific.h"
 #include "cc/resources/bitmap_content_layer_updater.h"
 #include "third_party/skia/include/core/SkBitmap.h"
 
 namespace cc {
 
 class CachingBitmapContentLayerUpdater : public BitmapContentLayerUpdater {
  public:
   static scoped_refptr<CachingBitmapContentLayerUpdater> Create(
       scoped_ptr<LayerPainter>,
       RenderingStatsInstrumentation* stats_instrumentation,
       int layer_id);
 
   virtual void PrepareToUpdate(gfx::Rect content_rect,
+                               gfx::Rect paint_rect,
                                gfx::Size tile_size,
                                float contents_width_scale,
                                float contents_height_scale,
                                gfx::Rect* resulting_opaque_rect) OVERRIDE;
 
   bool pixels_did_change() const {
     return pixels_did_change_;
   }
 
  private:
   CachingBitmapContentLayerUpdater(
       scoped_ptr<LayerPainter> painter,
       RenderingStatsInstrumentation* stats_instrumentation,
       int layer_id);
   virtual ~CachingBitmapContentLayerUpdater();
 
   bool pixels_did_change_;
   SkBitmap cached_bitmap_;
 
   DISALLOW_COPY_AND_ASSIGN(CachingBitmapContentLayerUpdater);
 };
 
 }  // namespace cc
 
 #endif  // CC_RESOURCES_CACHING_BITMAP_CONTENT_LAYER_UPDATER_H_