Dirty rects always contain full tiles with delegated rendering.

cc/layers/tiled_layer.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_LAYERS_TILED_LAYER_H_
 #define CC_LAYERS_TILED_LAYER_H_
 
 #include "cc/base/cc_export.h"
 #include "cc/layers/contents_scaling_layer.h"
 #include "cc/resources/layer_tiling_data.h"
@@ skipping 85 matching lines @@
                    int right,
                    int bottom,
                    ResourceUpdateQueue* queue,
                    const OcclusionTracker* occlusion,
                    bool* did_paint);
   bool HaveTexturesForTiles(int left,
                             int top,
                             int right,
                             int bottom,
                             bool ignore_occlusions);
-  gfx::Rect MarkTilesForUpdate(int left,
-                               int top,
-                               int right,
-                               int bottom,
-                               bool ignore_occlusions);
-  void UpdateTileTextures(gfx::Rect paint_rect,
+  void MarkTilesForUpdate(gfx::Rect* update_rect,
+                          gfx::Rect* paint_rect,
                           int left,
                           int top,
                           int right,
+                          int bottom,
+                          bool ignore_occlusions);
+  void UpdateTileTextures(gfx::Rect update_rect,
+                          gfx::Rect paint_rect,
+                          int left,
+                          int top,
+                          int right,
                           int bottom,
                           ResourceUpdateQueue* queue,
                           const OcclusionTracker* occlusion);
   void UpdateScrollPrediction();
 
   UpdatableTile* TileAt(int i, int j) const;
   UpdatableTile* CreateTile(int i, int j);
 
   bool IsSmallAnimatedLayer() const;
 
   ResourceFormat texture_format_;
   bool skips_draw_;
   bool failed_update_;
 
   // Used for predictive painting.
   gfx::Vector2d predicted_scroll_;
   gfx::Rect predicted_visible_rect_;
   gfx::Rect previous_visible_rect_;
   gfx::Size previous_content_bounds_;
 
   TilingOption tiling_option_;
   scoped_ptr<LayerTilingData> tiler_;
 
   DISALLOW_COPY_AND_ASSIGN(TiledLayer);
 };
 
 }  // namespace cc
 
 #endif  // CC_LAYERS_TILED_LAYER_H_