GSB uses delta_hints to calculate scrolling chain.

content/shell/test_runner/event_sender.cc

Index: content/shell/test_runner/event_sender.cc
diff --git a/content/shell/test_runner/event_sender.cc b/content/shell/test_runner/event_sender.cc
index 02ece2fb7ec769e0b7677631844de2ee87fdb573..dfb6cc3d7f5341687abdaf35bd71a10b974e08fc 100644
--- a/content/shell/test_runner/event_sender.cc
+++ b/content/shell/test_runner/event_sender.cc
@@ -2364,6 +2364,9 @@ void EventSender::GestureEvent(WebInputEvent::Type type, gin::Arguments* args) {
       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: