Make LayerScrollOffsetDelegate updates consistent.

cc/layers/layer_impl.h

Index: cc/layers/layer_impl.h
diff --git a/cc/layers/layer_impl.h b/cc/layers/layer_impl.h
index 16747aa9a5bc258a58f854d10b38aad737c22e2d..99642cb2a26f7d3701a0a01d6c63693fc27ad109 100644
--- a/cc/layers/layer_impl.h
+++ b/cc/layers/layer_impl.h
@@ -65,6 +65,15 @@ enum DrawMode {
 class CC_EXPORT LayerImpl : public LayerAnimationValueObserver,
                             public LayerAnimationValueProvider {
  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::Vector2dF& new_value) = 0;
+    virtual gfx::Vector2dF GetTotalScrollOffset() = 0;
+    virtual bool IsExternalFlingActive() const = 0;
+  };
+
   typedef LayerImplList RenderSurfaceListType;
   typedef LayerImplList LayerListType;
   typedef RenderSurfaceImpl RenderSurfaceType;
@@ -361,8 +370,7 @@ class CC_EXPORT LayerImpl : public LayerAnimationValueObserver,
                                       float* contents_scale_y,
                                       gfx::Size* content_bounds);
 
-  void SetScrollOffsetDelegate(
-      LayerScrollOffsetDelegate* scroll_offset_delegate);
+  void SetScrollOffsetDelegate(ScrollOffsetDelegate* scroll_offset_delegate);
   bool IsExternalFlingActive() const;
 
   void SetScrollOffset(const gfx::Vector2d& scroll_offset);
@@ -594,7 +602,7 @@ class CC_EXPORT LayerImpl : public LayerAnimationValueObserver,
   gfx::Size bounds_;
   gfx::SizeF temporary_impl_bounds_;
   gfx::Vector2d scroll_offset_;
-  LayerScrollOffsetDelegate* scroll_offset_delegate_;
+  ScrollOffsetDelegate* scroll_offset_delegate_;
   LayerImpl* scroll_clip_layer_;
   bool scrollable_ : 1;
   bool should_scroll_on_main_thread_ : 1;