Dirty rects always contain full tiles with delegated rendering.

cc/resources/bitmap_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_BITMAP_CONTENT_LAYER_UPDATER_H_
 #define CC_RESOURCES_BITMAP_CONTENT_LAYER_UPDATER_H_
 
 #include "cc/base/cc_export.h"
 #include "cc/resources/content_layer_updater.h"
 #include "skia/ext/refptr.h"
@@ skipping 29 matching lines @@
   };
 
   static scoped_refptr<BitmapContentLayerUpdater> Create(
       scoped_ptr<LayerPainter> painter,
       RenderingStatsInstrumentation* stats_instrumenation,
       int layer_id);
 
   virtual scoped_ptr<LayerUpdater::Resource> CreateResource(
       PrioritizedResourceManager* manager) OVERRIDE;
   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;
   void UpdateTexture(ResourceUpdateQueue* queue,
                      PrioritizedResource* resource,
                      gfx::Rect source_rect,
                      gfx::Vector2d dest_offset,
                      bool partial_update);
   virtual void SetOpaque(bool opaque) OVERRIDE;
@@ skipping 11 matching lines @@
   gfx::Size canvas_size_;
   bool opaque_;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(BitmapContentLayerUpdater);
 };
 
 }  // namespace cc
 
 #endif  // CC_RESOURCES_BITMAP_CONTENT_LAYER_UPDATER_H_