GSB uses delta_hints to calculate scrolling chain.

cc/input/scroll_state_data.h

 // Copyright 2015 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_SCROLL_STATE_DATA_H_
 #define CC_INPUT_SCROLL_STATE_DATA_H_
 
 #include <stdint.h>
 
 #include <list>
 
 #include "cc/cc_export.h"
 #include "cc/trees/property_tree.h"
 
 namespace cc {
 
 class CC_EXPORT ScrollStateData {
  public:
   ScrollStateData();
   ScrollStateData(const ScrollStateData& other);
 
   // Scroll delta in viewport coordinates (DIP).
   double delta_x;
   double delta_y;
+  // Scroll delta hint in viewport coordinates (DIP).
+  // Delta hints are equal to deltas of the first gesture scroll update event in
+  // a scroll sequence and are used for hittesting.
+  double delta_x_hint;
+  double delta_y_hint;
   // Pointer (i.e. cursor/touch point) position in viewport coordinates (DIP).
   int position_x;
   int position_y;
   // Scroll velocity in DIP/seconds.
   double velocity_x;
   double velocity_y;
 
   bool is_beginning;
   bool is_in_inertial_phase;
   bool is_ending;
@@ skipping 33 matching lines @@
   // The last scroll node to respond to a scroll, or null if none exists.
   ScrollNode* current_native_scrolling_node_;
   // The id of the last native element to respond to a scroll, or 0 if none
   // exists.
   ElementId current_native_scrolling_element_;
 };
 
 }  // namespace cc
 
 #endif  // CC_INPUT_SCROLL_STATE_DATA_H_