Only call UpdateScrollOffsetDelegate once on setting delegate.

cc/layers/layer_impl.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_LAYER_IMPL_H_
 #define CC_LAYERS_LAYER_IMPL_H_
 
 #include <set>
 #include <string>
 
@@ skipping 64 matching lines @@
                             public LayerAnimationValueProvider,
                             public AnimationDelegate {
  public:
   // Allows for the ownership of the total scroll offset to be delegated outside
   // of the layer.
   class ScrollOffsetDelegate {
    public:
     virtual void SetTotalScrollOffset(const gfx::ScrollOffset& new_value) = 0;
     virtual gfx::ScrollOffset GetTotalScrollOffset() = 0;
     virtual bool IsExternalFlingActive() const = 0;
+    virtual void Update() const = 0;
   };
 
   typedef LayerImplList RenderSurfaceListType;
   typedef LayerImplList LayerListType;
   typedef RenderSurfaceImpl RenderSurfaceType;
 
   enum RenderingContextConstants { NO_RENDERING_CONTEXT = 0 };
 
   static scoped_ptr<LayerImpl> Create(LayerTreeImpl* tree_impl, int id) {
     return make_scoped_ptr(new LayerImpl(tree_impl, id));
@@ skipping 622 matching lines @@
   DrawProperties<LayerImpl> draw_properties_;
 
   scoped_refptr<base::debug::ConvertableToTraceFormat> debug_info_;
 
   DISALLOW_COPY_AND_ASSIGN(LayerImpl);
 };
 
 }  // namespace cc
 
 #endif  // CC_LAYERS_LAYER_IMPL_H_