GSB uses delta_hints to calculate scrolling chain.

third_party/WebKit/Source/web/tests/RootScrollerTest.cpp

 // Copyright 2016 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 "bindings/core/v8/NodeOrString.h"
 #include "core/dom/ClientRect.h"
 #include "core/exported/WebRemoteFrameImpl.h"
 #include "core/frame/BrowserControls.h"
 #include "core/frame/FrameTestHelpers.h"
 #include "core/frame/LocalFrameView.h"
@@ skipping 133 matching lines @@
                                               int delta_x,
                                               int delta_y) {
     WebGestureEvent event(type, WebInputEvent::kNoModifiers,
                           WebInputEvent::kTimeStampForTesting);
     event.source_device = device;
     event.x = 100;
     event.y = 100;
     if (type == WebInputEvent::kGestureScrollUpdate) {
       event.data.scroll_update.delta_x = delta_x;
       event.data.scroll_update.delta_y = delta_y;
+    } else if (type == WebInputEvent::kGestureScrollBegin) {
+      event.data.scroll_begin.delta_x_hint = delta_x;
+      event.data.scroll_begin.delta_y_hint = delta_y;
     }
     return WebCoalescedInputEvent(event);
   }
 
   WebViewBase* InitializeInternal(const std::string& url,
                                   FrameTestHelpers::TestWebViewClient* client) {
     RuntimeEnabledFeatures::SetSetRootScrollerEnabled(true);
 
     helper_.InitializeAndLoad(url, nullptr, client, nullptr,
                               &ConfigureSettings);
@@ skipping 571 matching lines @@
     widget = local_frame->FrameWidget();
     widget->Resize(WebSize(400, 400));
   }
 
   Document* document = local_frame->GetFrameView()->GetFrame().GetDocument();
   Element* container = document->getElementById("container");
 
   // Try scrolling in the iframe.
   {
     widget->HandleInputEvent(
-        GenerateWheelGestureEvent(WebInputEvent::kGestureScrollBegin));
+        GenerateWheelGestureEvent(WebInputEvent::kGestureScrollBegin, 0, -100));
     widget->HandleInputEvent(GenerateWheelGestureEvent(
         WebInputEvent::kGestureScrollUpdate, 0, -100));
     widget->HandleInputEvent(
         GenerateWheelGestureEvent(WebInputEvent::kGestureScrollEnd));
     EXPECT_EQ(100, container->scrollTop());
   }
 
   // Set the container Element as the root scroller.
   {
     NonThrowableExceptionState non_throw;
     document->setRootScroller(container, non_throw);
     EXPECT_EQ(container, document->rootScroller());
   }
 
   // Try scrolling in the iframe now that it has a root scroller set.
   {
     widget->HandleInputEvent(
-        GenerateWheelGestureEvent(WebInputEvent::kGestureScrollBegin));
+        GenerateWheelGestureEvent(WebInputEvent::kGestureScrollBegin, 0, -100));
     widget->HandleInputEvent(GenerateWheelGestureEvent(
         WebInputEvent::kGestureScrollUpdate, 0, -100));
     widget->HandleInputEvent(
         GenerateWheelGestureEvent(WebInputEvent::kGestureScrollEnd));
 
     // TODO(bokan): This doesn't work right now because we notice in
     // Element::nativeApplyScroll that the container is the
     // effectiveRootScroller but the only way we expect to get to
     // nativeApplyScroll is if the effective scroller had its applyScroll
     // ViewportScrollCallback removed.  Keep the scrolls to guard crashes
@@ skipping 469 matching lines @@
   // Ensure the target and container weren't put into the same layer.
   ASSERT_NE(ToLayoutBox(target->GetLayoutObject())->EnclosingLayer(),
             ToLayoutBox(container->GetLayoutObject())->Layer());
 
   CheckHitTestAtBottomOfScreen();
 }
 
 }  // namespace
 
 }  // namespace blink