MouseWheelEventManager is added to handle wheel events.

third_party/WebKit/Source/core/input/BUILD.gn

 # 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.
 
 import("//third_party/WebKit/Source/core/core.gni")
 
 blink_core_sources("input") {
   sources = [
     "BoundaryEventDispatcher.cpp",
     "BoundaryEventDispatcher.h",
     "ContextMenuAllowedScope.cpp",
     "ContextMenuAllowedScope.h",
     "EventHandler.cpp",
     "EventHandler.h",
     "EventHandlingUtil.cpp",
     "EventHandlingUtil.h",
     "GestureManager.cpp",
     "GestureManager.h",
     "InputDeviceCapabilities.cpp",
     "InputDeviceCapabilities.h",
     "KeyboardEventManager.cpp",
     "KeyboardEventManager.h",
     "MouseEventManager.cpp",
     "MouseEventManager.h",
+    "MouseWheelEventManager.cpp",
+    "MouseWheelEventManager.h",
     "PointerEventManager.cpp",
     "PointerEventManager.h",
     "ScrollManager.cpp",
     "ScrollManager.h",
     "TouchActionUtil.cpp",
     "TouchActionUtil.h",
     "TouchEventManager.cpp",
     "TouchEventManager.h",
   ]
 
   configs += [
     # TODO(jschuh): crbug.com/167187 fix size_t to int truncations.
     "//build/config/compiler:no_size_t_to_int_warning",
   ]
 }