Create MouseWheelEvent directly instead of through MouseEvent.

ui/events/event.h

 // 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.
 
 #ifndef UI_EVENTS_EVENT_H_
 #define UI_EVENTS_EVENT_H_
 
 #include <stdint.h>
 
 #include <memory>
@@ skipping 467 matching lines @@
              EventType type,
              int flags)
       : LocatedEvent(model, source, target),
         changed_button_flags_(model.changed_button_flags_),
         pointer_details_(model.pointer_details_) {
     SetType(type);
     set_flags(flags);
   }
 
   // Used for synthetic events in testing, gesture recognizer and Ozone
+  // Note: Use the ctor for MouseWheelEvent if type is ET_MOUSEWHEEL.
   MouseEvent(EventType type,
              const gfx::Point& location,
              const gfx::Point& root_location,
              base::TimeTicks time_stamp,
              int flags,
              int changed_button_flags);
 
   // Conveniences to quickly test what button is down
   bool IsOnlyLeftMouseButton() const {
     return button_flags() == EF_LEFT_MOUSE_BUTTON;
@@ skipping 518 matching lines @@
   // dispatched. This field gets a non-zero value only for gestures that are
   // released through TouchDispositionGestureFilter::SendGesture. The gesture
   // events that aren't fired directly in response to processing a touch-event
   // (e.g. timer fired ones), this id is zero. See crbug.com/618738.
   uint32_t unique_touch_event_id_;
 };
 
 }  // namespace ui
 
 #endif  // UI_EVENTS_EVENT_H_