GSB uses delta_hints to calculate scrolling chain.

cc/input/input_handler.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_INPUT_INPUT_HANDLER_H_
 #define CC_INPUT_INPUT_HANDLER_H_
 
 #include <memory>
 
 #include "base/macros.h"
@@ skipping 113 matching lines @@
   virtual ScrollStatus ScrollBegin(ScrollState* scroll_state,
                                    ScrollInputType type) = 0;
 
   // Similar to ScrollBegin, except the hit test is skipped and scroll always
   // targets at the root layer.
   virtual ScrollStatus RootScrollBegin(ScrollState* scroll_state,
                                        ScrollInputType type) = 0;
 
   // Returns SCROLL_ON_IMPL_THREAD if a layer is actively being scrolled or
   // a subsequent call to ScrollAnimated can begin on the impl thread.
-  virtual ScrollStatus ScrollAnimatedBegin(
-      const gfx::Point& viewport_point) = 0;
+  virtual ScrollStatus ScrollAnimatedBegin(ScrollState* scroll_state) = 0;
 
   // Returns SCROLL_ON_IMPL_THREAD if an animation is initiated on the impl
   // thread. delayed_by is the delay that is taken into account when determining
   // the duration of the animation.
   virtual ScrollStatus ScrollAnimated(const gfx::Point& viewport_point,
                                       const gfx::Vector2dF& scroll_delta,
                                       base::TimeDelta delayed_by) = 0;
 
   // Scroll the layer selected by |ScrollBegin| by given |scroll_state| delta.
   // Internally, the delta is transformed to local layer's coordinate space for
@@ skipping 78 matching lines @@
   InputHandler() {}
   virtual ~InputHandler() {}
 
  private:
   DISALLOW_COPY_AND_ASSIGN(InputHandler);
 };
 
 }  // namespace cc
 
 #endif  // CC_INPUT_INPUT_HANDLER_H_