Remove PageVisibilityObserver contextDestroyed() notifications.

third_party/WebKit/Source/core/workers/WorkerThreadLifecycleObserver.h

 // Copyright 2016 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 WorkerThreadLifecycleObserver_h
 #define WorkerThreadLifecycleObserver_h
 
 #include "core/CoreExport.h"
 #include "platform/LifecycleObserver.h"
 
@@ skipping 11 matching lines @@
 // the worker thread is about to terminate. While contextDestroyed() is called,
 // it is guaranteed that the worker thread is still alive.
 //
 // A newly created observer should firstly check whether the worker thread is
 // alive by wasContextDestroyedBeforeObserverCreation(). If this return true,
 // the worker thread has already been terminated before the observer is created,
 // and contextDestroyed() is never notified.
 class CORE_EXPORT WorkerThreadLifecycleObserver
     : public LifecycleObserver<WorkerThreadLifecycleContext,
                                WorkerThreadLifecycleObserver> {
+ public:
+  virtual void contextDestroyed(WorkerThreadLifecycleContext*) {}
+
  protected:
   explicit WorkerThreadLifecycleObserver(WorkerThreadLifecycleContext*);
   virtual ~WorkerThreadLifecycleObserver();
 
   bool wasContextDestroyedBeforeObserverCreation() const {
     return m_wasContextDestroyedBeforeObserverCreation;
   }
 
  private:
   const bool m_wasContextDestroyedBeforeObserverCreation;
 };
 
 }  // namespace blink
 
 #endif  // WorkerThreadLifecycleObserver_h