GSB uses delta_hints to calculate scrolling chain.

ui/events/blink/input_handler_proxy.cc

 // Copyright 2013 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 "ui/events/blink/input_handler_proxy.h"
 
 #include <stddef.h>
 
 #include <algorithm>
 
@@ skipping 123 matching lines @@
   scroll_begin_event.data.scroll_begin.delta_y_hint = 0;
   return scroll_begin_event;
 }
 
 cc::ScrollState CreateScrollStateForGesture(const WebGestureEvent& event) {
   cc::ScrollStateData scroll_state_data;
   switch (event.GetType()) {
     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.
       scroll_state_data.is_in_inertial_phase =
           (event.data.scroll_begin.inertial_phase ==
            WebGestureEvent::kMomentumPhase);
       break;
     case WebInputEvent::kGestureFlingStart:
       scroll_state_data.velocity_x = event.data.fling_start.velocity_x;
       scroll_state_data.velocity_y = event.data.fling_start.velocity_y;
@@ skipping 664 matching lines @@
       blink::WebGestureEvent::ScrollUnits::kPage) {
     scroll_status.thread = cc::InputHandler::SCROLL_ON_MAIN_THREAD;
     scroll_status.main_thread_scrolling_reasons =
         cc::MainThreadScrollingReason::kContinuingMainThreadScroll;
   } else if (gesture_event.data.scroll_begin.target_viewport) {
     scroll_status = input_handler_->RootScrollBegin(
         &scroll_state, GestureScrollInputType(gesture_event.source_device));
   } 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));
   }
   UMA_HISTOGRAM_ENUMERATION("Renderer4.CompositorScrollHitTestResult",
                             scroll_status.thread,
                             cc::InputHandler::LAST_SCROLL_STATUS + 1);
 
   RecordMainThreadScrollingReasons(gesture_event.source_device,
                                    scroll_status.main_thread_scrolling_reasons);
@@ skipping 804 matching lines @@
                  scroll_elasticity_controller_->GetWeakPtr(), gesture_event,
                  scroll_result));
 }
 
 void InputHandlerProxy::SetTickClockForTesting(
     std::unique_ptr<base::TickClock> tick_clock) {
   tick_clock_ = std::move(tick_clock);
 }
 
 }  // namespace ui