GSB uses delta_hints to calculate scrolling chain.

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

Index: third_party/WebKit/Source/web/tests/RootScrollerTest.cpp
diff --git a/third_party/WebKit/Source/web/tests/RootScrollerTest.cpp b/third_party/WebKit/Source/web/tests/RootScrollerTest.cpp
index 2ef18d8214daa189afacb5450752ef7d595d5027..e2266e763c91b7f2ee5f239ebde9b42468a38165 100644
--- a/third_party/WebKit/Source/web/tests/RootScrollerTest.cpp
+++ b/third_party/WebKit/Source/web/tests/RootScrollerTest.cpp
@@ -151,6 +151,9 @@ class RootScrollerTest : public ::testing::Test {
     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);
   }
@@ -709,7 +712,7 @@ TEST_F(RootScrollerTest, MAYBE_RemoteMainFrame) {
   // Try scrolling in the iframe.
   {
     widget->HandleInputEvent(
-        GenerateWheelGestureEvent(WebInputEvent::kGestureScrollBegin));
+        GenerateWheelGestureEvent(WebInputEvent::kGestureScrollBegin, 0, -100));
     widget->HandleInputEvent(GenerateWheelGestureEvent(
         WebInputEvent::kGestureScrollUpdate, 0, -100));
     widget->HandleInputEvent(
@@ -727,7 +730,7 @@ TEST_F(RootScrollerTest, MAYBE_RemoteMainFrame) {
   // 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(