Remove PageVisibilityObserver contextDestroyed() notifications.

third_party/WebKit/Source/core/dom/ContextLifecycleObserver.h

 /*
  * Copyright (C) 2008 Apple 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
@@ skipping 45 matching lines @@
  private:
   WeakMember<ExecutionContext> m_executionContext;
 };
 
 // ContextLifecycleObserver provides an additional contextDestroyed() hook
 // to execute cleanup code when a context is destroyed. Prefer the simpler
 // ContextClient when possible.
 class CORE_EXPORT ContextLifecycleObserver
     : public LifecycleObserver<ExecutionContext, ContextLifecycleObserver> {
  public:
+  virtual void contextDestroyed(ExecutionContext*) {}
+
   ExecutionContext* getExecutionContext() const { return lifecycleContext(); }
   LocalFrame* frame() const;
 
   enum Type {
     GenericType,
     SuspendableObjectType,
   };
 
   Type observerType() const { return m_observerType; }
 
@@ skipping 36 matching lines @@
   explicit DOMWindowClient(LocalDOMWindow*);
   explicit DOMWindowClient(LocalFrame*);
 
  private:
   WeakMember<LocalDOMWindow> m_domWindow;
 };
 
 }  // namespace blink
 
 #endif  // ContextLifecycleObserver_h