Dirty rects always contain full tiles with delegated rendering.

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"
@@ skipping 14 matching lines @@
   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::Rect paint_rect,
                      float contents_width_scale,
                      float contents_height_scale,
                      gfx::Rect* resulting_opaque_rect);
   gfx::Rect content_rect() const { return content_rect_; }
+  gfx::Rect paint_rect() const { return paint_rect_; }
 
   bool layer_is_opaque() const { return layer_is_opaque_; }
 
   RenderingStatsInstrumentation* rendering_stats_instrumentation_;
   int layer_id_;
 
+  // True when it is known that all output pixels will be opaque.
+  bool layer_is_opaque_;
+
  private:
   gfx::Rect content_rect_;
+  gfx::Rect paint_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_