Remove PageVisibilityObserver contextDestroyed() notifications.

third_party/WebKit/Source/platform/LifecycleObserver.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 21 matching lines @@
 namespace blink {
 
 template <typename Context, typename Observer>
 class LifecycleNotifier;
 
 template <typename Context, typename Observer>
 class LifecycleObserver : public GarbageCollectedMixin {
  public:
   DEFINE_INLINE_VIRTUAL_TRACE() { visitor->trace(m_lifecycleContext); }
 
-  virtual void contextDestroyed(Context*) {}
-
   Context* lifecycleContext() const { return m_lifecycleContext; }
 
   void clearContext() { setContext(nullptr); }
 
  protected:
   explicit LifecycleObserver(Context* context) : m_lifecycleContext(nullptr) {
     setContext(context);
   }
 
   void setContext(Context*);
@@ skipping 15 matching lines @@
 
   if (m_lifecycleContext) {
     static_cast<Notifier*>(m_lifecycleContext)
         ->addObserver(static_cast<Observer*>(this));
   }
 }
 
 }  // namespace blink
 
 #endif  // LifecycleObserver_h