Create MouseWheelEvent directly instead of through MouseEvent.

ui/events/event.cc

 // Copyright (c) 2012 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/event.h"
 
 #include <utility>
 
 #include "base/memory/ptr_util.h"
 
@@ skipping 504 matching lines @@
                        base::TimeTicks time_stamp,
                        int flags,
                        int changed_button_flags)
     : LocatedEvent(type,
                    gfx::PointF(location),
                    gfx::PointF(root_location),
                    time_stamp,
                    flags),
       changed_button_flags_(changed_button_flags),
       pointer_details_(PointerDetails(EventPointerType::POINTER_TYPE_MOUSE)) {
+  DCHECK_NE(ET_MOUSEWHEEL, type);
   latency()->AddLatencyNumber(INPUT_EVENT_LATENCY_UI_COMPONENT, 0, 0);
   if (this->type() == ET_MOUSE_MOVED && IsAnyButton())
     SetType(ET_MOUSE_DRAGGED);
 }
 
 // static
 bool MouseEvent::IsRepeatedClickEvent(
     const MouseEvent& event1,
     const MouseEvent& event2) {
   // These values match the Windows defaults.
@@ skipping 137 matching lines @@
       offset_(mouse_wheel_event.offset()) {
   DCHECK(type() == ET_MOUSEWHEEL);
 }
 
 MouseWheelEvent::MouseWheelEvent(const gfx::Vector2d& offset,
                                  const gfx::Point& location,
                                  const gfx::Point& root_location,
                                  base::TimeTicks time_stamp,
                                  int flags,
                                  int changed_button_flags)
-    : MouseEvent(ui::ET_MOUSEWHEEL,
+    : MouseEvent(ui::ET_UNKNOWN,
                  location,
                  root_location,
                  time_stamp,
                  flags,
                  changed_button_flags),
-      offset_(offset) {}
+      offset_(offset) {
+  // Set event type to ET_UNKNOWN initially in MouseEvent() to pass the
+  // DCHECK for type to enforce that we use MouseWheelEvent() to create
+  // a MouseWheelEvent.
+  SetType(ui::ET_MOUSEWHEEL);
+}
 
 #if defined(OS_WIN)
 // This value matches windows WHEEL_DELTA.
 // static
 const int MouseWheelEvent::kWheelDelta = 120;
 #else
 // This value matches GTK+ wheel scroll amount.
 const int MouseWheelEvent::kWheelDelta = 53;
 #endif
 
@@ skipping 600 matching lines @@
                    gfx::PointF(x, y),
                    time_stamp,
                    flags | EF_FROM_TOUCH),
       details_(details),
       unique_touch_event_id_(unique_touch_event_id) {}
 
 GestureEvent::~GestureEvent() {
 }
 
 }  // namespace ui