Reduce forward declarations in core/dom

Source/core/dom/ContextLifecycleNotifier.h

 /*
  * Copyright (C) 2008 Apple Inc. All Rights Reserved.
  * Copyright (C) 2013 Google Inc. All Rights Reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 17 matching lines @@
 #define ContextLifecycleNotifier_h
 
 #include "core/dom/ActiveDOMObject.h"
 #include "platform/LifecycleNotifier.h"
 #include "wtf/HashSet.h"
 #include "wtf/PassOwnPtr.h"
 
 namespace WebCore {
 
 class ActiveDOMObject;
-class ContextLifecycleObserver;
 class ExecutionContext;
 
 class ContextLifecycleNotifier : public LifecycleNotifier<ExecutionContext> {
 public:
     static PassOwnPtr<ContextLifecycleNotifier> create(ExecutionContext*);
 
     virtual ~ContextLifecycleNotifier();
 
     typedef HashSet<ActiveDOMObject*> ActiveDOMObjectSet;
 
@@ skipping 17 matching lines @@
 };
 
 inline PassOwnPtr<ContextLifecycleNotifier> ContextLifecycleNotifier::create(ExecutionContext* context)
 {
     return adoptPtr(new ContextLifecycleNotifier(context));
 }
 
 } // namespace WebCore
 
 #endif // ContextLifecycleNotifier_h