GSB uses delta_hints to calculate scrolling chain.

content/shell/test_runner/event_sender.cc

 // Copyright 2014 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.
 
 #include "content/shell/test_runner/event_sender.h"
 
 #include <stddef.h>
 
 #include <memory>
 
@@ skipping 2346 matching lines @@
       current_gesture_location_.x =
           current_gesture_location_.x + event.data.scroll_update.delta_x;
       current_gesture_location_.y =
           current_gesture_location_.y + event.data.scroll_update.delta_y;
       break;
     }
     case WebInputEvent::kGestureScrollBegin:
       current_gesture_location_ = WebPoint(x, y);
       event.x = current_gesture_location_.x;
       event.y = current_gesture_location_.y;
+      // TODO(sahel) : change this to get scroll delta hints from args instead
+      // of ignoring them. See crbug.com/728214.
+      event.data.scroll_begin.ignore_delta_hints = true;
       break;
     case WebInputEvent::kGestureScrollEnd:
     case WebInputEvent::kGestureFlingStart:
       event.x = current_gesture_location_.x;
       event.y = current_gesture_location_.y;
       break;
     case WebInputEvent::kGesturePinchBegin:
     case WebInputEvent::kGesturePinchEnd:
       current_gesture_location_ = WebPoint(x, y);
       event.x = current_gesture_location_.x;
@@ skipping 539 matching lines @@
   return view()->MainFrame()->ToWebLocalFrame()->FrameWidget();
 }
 
 std::unique_ptr<WebInputEvent> EventSender::TransformScreenToWidgetCoordinates(
     const WebInputEvent& event) {
   return delegate()->TransformScreenToWidgetCoordinates(
       web_widget_test_proxy_base_, event);
 }
 
 }  // namespace test_runner