ui: Cleanup class/struct forward declarations

ui/events/platform/x11/x11_hotplug_event_handler.h

 // Copyright 2014 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_PLATFORM_X11_X11_HOTPLUG_EVENT_HANDLER_H_
 #define UI_EVENTS_PLATFORM_X11_X11_HOTPLUG_EVENT_HANDLER_H_
 
 #include "base/macros.h"
 #include "ui/events/devices/x11/device_list_cache_x11.h"
 #include "ui/gfx/x/x11_atom_cache.h"
 
 namespace ui {
 
-class DeviceHotplugEventObserver;
-
 // Parses X11 native devices and propagates the list of active devices to an
 // observer.
 class X11HotplugEventHandler {
  public:
   X11HotplugEventHandler();
   ~X11HotplugEventHandler();
 
   // Called on an X11 hotplug event.
   void OnHotplugEvent();
 
  private:
   X11AtomCache atom_cache_;
 
   DISALLOW_COPY_AND_ASSIGN(X11HotplugEventHandler);
 };
 
 }  // namespace ui
 
 #endif  // UI_EVENTS_PLATFORM_X11_X11_HOTPLUG_EVENT_HANDLER_H_