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 1ed598061c2dca8f038386555d8b46bf85c0df09..6c86bc7a96b8a5d424df43df8ab12332a73db2c8 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;

[bokan, 2017/05/30 15:57:44]

The delta on a scroll_begin is called a hint, but for scroll_state I think we
could just put it in delta_x and delta_y without issue (and use
ignore_delta_hints here to decide whether to add it or not). WDYT?

[sahel, 2017/05/30 16:02:06]

I had this implementation initially, but ended up actually distributing these
delta and changing scroll offsets on GSB.

I can say that these are only hints but I thought this flag might get confused
by ignore_delta_hints flag. I can change it if you prefer to avoid separate
variables for delta hints.

[bokan, 2017/05/30 16:05:29]

Tim is the expert here so I'll defer to him but personally I think the "hint"
aspect is implied by the context of the scroll_state having is_beginning = true
(i.e. we know that a GSB doesn't *scroll* the delta it provides) so having a
separate delta_x/y is overly verbose. 

+      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;
       scroll_state_data.is_beginning = true;
       // On Mac, a GestureScrollBegin in the inertial phase indicates a fling
       // start.
@@ -825,8 +829,7 @@ 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));