GSB uses delta_hints to calculate scrolling chain.

cc/input/scroll_state_data.h

Index: cc/input/scroll_state_data.h
diff --git a/cc/input/scroll_state_data.h b/cc/input/scroll_state_data.h
index f3559a89d0758fe72bd43cfe0f2e3b1b964e0e6d..ba33e46626bc2028fc32c012a0591a6cc2b9cf03 100644
--- a/cc/input/scroll_state_data.h
+++ b/cc/input/scroll_state_data.h
@@ -22,6 +22,11 @@ class CC_EXPORT ScrollStateData {
   // 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;