Simplify lifecycle notifiers and observers.

Source/core/page/PageLifecycleNotifier.h

 /*
  * 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
  *    notice, this list of conditions and the following disclaimer in the
  *    documentation and/or other materials provided with the distribution.
  *
  * THIS SOFTWARE IS PROVIDED BY APPLE INC. ``AS IS'' AND ANY
  * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
  * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL APPLE INC. OR
  * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
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  * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
  * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY
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 #ifndef PageLifecycleNotifier_h
 #define PageLifecycleNotifier_h
 
 #include "platform/LifecycleNotifier.h"
-#include "wtf/HashSet.h"
-#include "wtf/PassOwnPtr.h"
-#include "wtf/TemporaryChange.h"
 
 namespace blink {
 
 class LocalFrame;
 class Page;
 class PageLifecycleObserver;
 
-class PageLifecycleNotifier : public LifecycleNotifier<Page> {
+class PageLifecycleNotifier : public LifecycleNotifier<Page, PageLifecycleObserver> {
 public:
+    void addObserver(PageLifecycleObserver*);
+    void removeObserver(PageLifecycleObserver*);
+
     void notifyPageVisibilityChanged();
     void notifyDidCommitLoad(LocalFrame*);
 
-    virtual void addObserver(Observer*) override;
-    virtual void removeObserver(Observer*) override;
-
 protected:
     explicit PageLifecycleNotifier(Page*);
-
-private:
-    using PageObserverSet = HashSet<PageLifecycleObserver*>;
-    PageObserverSet m_pageObservers;
 };
 
 } // namespace blink
 
 #endif // PageLifecycleNotifier_h