GSB uses delta_hints to calculate scrolling chain.

ui/events/blink/input_handler_proxy.cc

Index: ui/events/blink/input_handler_proxy.cc
diff --git a/ui/events/blink/input_handler_proxy.cc b/ui/events/blink/input_handler_proxy.cc
index 6ff1983d7139c16be4b7604726a8c80db40dea7f..2b1f46de9277e230d74ddeab4439a2fa53e3ee21 100644
--- a/ui/events/blink/input_handler_proxy.cc
+++ b/ui/events/blink/input_handler_proxy.cc
@@ -141,6 +141,10 @@ cc::ScrollState CreateScrollStateForGesture(const WebGestureEvent& event) {
     case WebInputEvent::kGestureScrollBegin:
       scroll_state_data.position_x = event.x;
       scroll_state_data.position_y = event.y;
+      scroll_state_data.delta_x_hint = -event.data.scroll_begin.delta_x_hint;
+      scroll_state_data.delta_y_hint = -event.data.scroll_begin.delta_y_hint;
+      scroll_state_data.ignore_delta_hints =
+          event.data.scroll_begin.ignore_delta_hints;

[bokan, 2017/06/19 21:43:17]

Do you know what this flag is used for? Does it ever get used in production or
is it just for tests?

[sahel, 2017/06/22 16:34:20]

I will get rid of this once I fix the event_sender to get delta_hints in
layouttests.

[bokan, 2017/06/22 21:44:37]

Acknowledged.

       scroll_state_data.is_beginning = true;
       // On Mac, a GestureScrollBegin in the inertial phase indicates a fling
       // start.
@@ -826,8 +830,7 @@ InputHandlerProxy::EventDisposition InputHandlerProxy::HandleGestureScrollBegin(
   } else if (ShouldAnimate(gesture_event.data.scroll_begin.delta_hint_units !=
                            blink::WebGestureEvent::ScrollUnits::kPixels)) {
     DCHECK(!scroll_state.is_in_inertial_phase());
-    gfx::Point scroll_point(gesture_event.x, gesture_event.y);
-    scroll_status = input_handler_->ScrollAnimatedBegin(scroll_point);
+    scroll_status = input_handler_->ScrollAnimatedBegin(&scroll_state);
   } else {
     scroll_status = input_handler_->ScrollBegin(
         &scroll_state, GestureScrollInputType(gesture_event.source_device));