ui: Cleanup class/struct forward declarations

ui/events/platform/platform_event_source.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_PLATFORM_EVENT_SOURCE_H_
 #define UI_EVENTS_PLATFORM_PLATFORM_EVENT_SOURCE_H_
 
 #include <stdint.h>
 
 #include <map>
 #include <memory>
 #include <vector>
 
 #include "base/auto_reset.h"
 #include "base/macros.h"
 #include "base/observer_list.h"
 #include "ui/events/events_export.h"
 #include "ui/events/platform/platform_event_types.h"
 
 namespace ui {
 
-class Event;
 class PlatformEventDispatcher;
 class PlatformEventObserver;
 class ScopedEventDispatcher;
 
 namespace test {
 class PlatformEventSourceTestAPI;
 }
 
 // PlatformEventSource receives events from a source and dispatches the events
 // to the appropriate dispatchers.
@@ skipping 70 matching lines @@
   bool overridden_dispatcher_restored_;
 
   base::ObserverList<PlatformEventObserver> observers_;
 
   DISALLOW_COPY_AND_ASSIGN(PlatformEventSource);
 };
 
 }  // namespace ui
 
 #endif  // UI_EVENTS_PLATFORM_PLATFORM_EVENT_SOURCE_H_