Refactor delegated scrolling in LayerImpl

cc/trees/layer_tree_impl.h

Index: cc/trees/layer_tree_impl.h
diff --git a/cc/trees/layer_tree_impl.h b/cc/trees/layer_tree_impl.h
index 8ab42634cc835c209f37da7d24623a93248e5b30..1d3367e0e00d5c5c1fbc9e3b15b46647d1ac6be6 100644
--- a/cc/trees/layer_tree_impl.h
+++ b/cc/trees/layer_tree_impl.h
@@ -33,7 +33,8 @@ class ContextProvider;
 class DebugRectHistory;
 class FrameRateCounter;
 class HeadsUpDisplayLayerImpl;
-class LayerScrollOffsetDelegateProxy;
+class LayerExternalScrollOffsetListener;
+class LayerScrollOffsetDelegate;
 class LayerTreeDebugState;
 class LayerTreeImpl;
 class LayerTreeSettings;
@@ -138,6 +139,7 @@ class CC_EXPORT LayerTreeImpl {
     hud_layer_ = layer_impl;
   }
 
+  // This function may return NULL, it is the caller's responsibility to check.
   LayerImpl* InnerViewportScrollLayer() const;
   // This function may return NULL, it is the caller's responsibility to check.
   LayerImpl* OuterViewportScrollLayer() const;
@@ -263,9 +265,10 @@ class CC_EXPORT LayerTreeImpl {
 
   void SetRootLayerScrollOffsetDelegate(
       LayerScrollOffsetDelegate* root_layer_scroll_offset_delegate);
-  void OnRootLayerDelegatedScrollOffsetChanged();
-  void UpdateScrollOffsetDelegate();
-  gfx::ScrollOffset GetDelegatedScrollOffset(LayerImpl* layer);
+  void UpdateRootScrollOffsetDelegate();
+  // Distribute the rool scroll between outer and inner viewport scroll layer.
+  // The outer viewport scroll layer scrolls first.
+  void DistributeRootScrollOffset();
 
   // Call this function when you expect there to be a swap buffer.
   // See swap_promise.h for how to use SwapPromise.
@@ -355,10 +358,7 @@ class CC_EXPORT LayerTreeImpl {
   HeadsUpDisplayLayerImpl* hud_layer_;
   LayerImpl* currently_scrolling_layer_;
   LayerScrollOffsetDelegate* root_layer_scroll_offset_delegate_;
-  scoped_ptr<LayerScrollOffsetDelegateProxy>
-      inner_viewport_scroll_delegate_proxy_;
-  scoped_ptr<LayerScrollOffsetDelegateProxy>
-      outer_viewport_scroll_delegate_proxy_;
+  scoped_ptr<LayerExternalScrollOffsetListener> layer_scroll_offset_listener_;
   SkColor background_color_;
   bool has_transparent_background_;