GSB uses delta_hints to calculate scrolling chain.

third_party/WebKit/public/platform/WebGestureEvent.h

 // Copyright 2016 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 WebGestureEvent_h
 #define WebGestureEvent_h
 
 #include "WebFloatSize.h"
 #include "WebGestureDevice.h"
 #include "WebInputEvent.h"
@@ skipping 67 matching lines @@
       float height;
     } long_press;
 
     struct {
       float first_finger_width;
       float first_finger_height;
     } two_finger_tap;
 
     struct {
       // Initial motion that triggered the scroll.
-      // May be redundant with deltaX/deltaY in the first scrollUpdate.
       float delta_x_hint;
       float delta_y_hint;
       // Default initialized to ScrollUnits::PrecisePixels.
       ScrollUnits delta_hint_units;
       // If true, this event will skip hit testing to find a scroll
       // target and instead just scroll the viewport.
       bool target_viewport;
       // The state of inertial phase scrolling. OSX has unique phases for normal
       // and momentum scroll events. Should always be UnknownMomentumPhase for
       // touch based input as it generates GestureFlingStart instead.
@@ skipping 142 matching lines @@
         return false;
     }
   }
 };
 
 #pragma pack(pop)
 
 }  // namespace blink
 
 #endif  // WebGestureEvent_h