Turn off computation of the interest rect in cc in synchronized paint mode.

cc/layers/content_layer_client.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_LAYERS_CONTENT_LAYER_CLIENT_H_
 #define CC_LAYERS_CONTENT_LAYER_CLIENT_H_
 
 #include "cc/base/cc_export.h"
 #include "cc/playback/display_item_list.h"
 
 namespace gfx {
 class Rect;
 }
 
 namespace cc {
 
 class CC_EXPORT ContentLayerClient {
  public:
   enum PaintingControlSetting {
     PAINTING_BEHAVIOR_NORMAL,
     DISPLAY_LIST_CONSTRUCTION_DISABLED,
     DISPLAY_LIST_CACHING_DISABLED,
     DISPLAY_LIST_PAINTING_DISABLED
   };
 
+  virtual gfx::Rect PaintableRegion() = 0;
+
   virtual scoped_refptr<DisplayItemList> PaintContentsToDisplayList(
       const gfx::Rect& clip,
       PaintingControlSetting painting_status) = 0;
 
   // If true the layer may skip clearing the background before rasterizing,
   // because it will cover any uncleared data with content.
   virtual bool FillsBoundsCompletely() const = 0;
 
   // Returns an estimate of the current memory usage within this object,
   // excluding memory shared with painting artifacts (i.e.,
   // DisplayItemList). Should be invoked after PaintContentsToDisplayList,
   // so that the result includes data cached internally during painting.
   virtual size_t GetApproximateUnsharedMemoryUsage() const = 0;
 
  protected:
   virtual ~ContentLayerClient() {}
 };
 
 }  // namespace cc
 
 #endif  // CC_LAYERS_CONTENT_LAYER_CLIENT_H_