Refactor delegated scrolling in LayerImpl

cc/input/layer_scroll_offset_delegate.h

 // Copyright 2013 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_INPUT_LAYER_SCROLL_OFFSET_DELEGATE_H_
 #define CC_INPUT_LAYER_SCROLL_OFFSET_DELEGATE_H_
 
 #include "base/basictypes.h"
 #include "ui/gfx/geometry/scroll_offset.h"
 #include "ui/gfx/geometry/size_f.h"
@@ skipping 24 matching lines @@
   // |min_page_scale_factor| page scale lower limit,
   // |max_page_scale_factor| page scale upper limit.
   virtual void UpdateRootLayerState(
       const gfx::ScrollOffset& total_scroll_offset,
       const gfx::ScrollOffset& max_scroll_offset,
       const gfx::SizeF& scrollable_size,
       float page_scale_factor,
       float min_page_scale_factor,
       float max_page_scale_factor) = 0;
 
+  virtual void SetBlockUpdates(bool block_updates) {}
   // This is called by the compositor to check whether a delegate-managed fling
   // is active or not.
   virtual bool IsExternalFlingActive() const = 0;
 
  protected:
   LayerScrollOffsetDelegate() {}
   virtual ~LayerScrollOffsetDelegate() {}
 
  private:
   DISALLOW_COPY_AND_ASSIGN(LayerScrollOffsetDelegate);
 };
 
 }  // namespace cc
 
 #endif  // CC_INPUT_LAYER_SCROLL_OFFSET_DELEGATE_H_