GSB uses delta_hints to calculate scrolling chain.

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

Index: third_party/WebKit/Source/web/tests/BrowserControlsTest.cpp
diff --git a/third_party/WebKit/Source/web/tests/BrowserControlsTest.cpp b/third_party/WebKit/Source/web/tests/BrowserControlsTest.cpp
index 8221e8160de095b4b940780a55621a5c43d04857..78ab421d8e8d9bfb8705ebc2955a3beb028311f4 100644
--- a/third_party/WebKit/Source/web/tests/BrowserControlsTest.cpp
+++ b/third_party/WebKit/Source/web/tests/BrowserControlsTest.cpp
@@ -108,13 +108,16 @@ class BrowserControlsTest : 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);
   }
 
   void VerticalScroll(float delta_y) {
     GetWebView()->HandleInputEvent(
-        GenerateEvent(WebInputEvent::kGestureScrollBegin));
+        GenerateEvent(WebInputEvent::kGestureScrollBegin, 0, delta_y));
     GetWebView()->HandleInputEvent(
         GenerateEvent(WebInputEvent::kGestureScrollUpdate, 0, delta_y));
     GetWebView()->HandleInputEvent(