Reland of Clear LifecycleObserver::m_context when LifecycleObserver::contextDestroyed gets called

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 25 matching lines @@
 public:
     DEFINE_INLINE_VIRTUAL_TRACE()
     {
         visitor->trace(m_lifecycleContext);
     }
 
     virtual void contextDestroyed() { }
 
     Context* lifecycleContext() const { return m_lifecycleContext; }
 
+    void clearContext()
+    {
+        setContext(nullptr);
+    }
+
 protected:
     explicit LifecycleObserver(Context* context)
         : m_lifecycleContext(nullptr)
     {
         setContext(context);
     }
 
     void setContext(Context*);
 
-    void clearContext()
-    {
-        setContext(nullptr);
-    }
-
 private:
     WeakMember<Context> m_lifecycleContext;
 };
 
 template<typename Context, typename Observer>
 inline void LifecycleObserver<Context, Observer>::setContext(Context* context)
 {
     if (m_lifecycleContext)
         m_lifecycleContext->removeObserver(static_cast<Observer*>(this));
 
     m_lifecycleContext = context;
 
     if (m_lifecycleContext)
         m_lifecycleContext->addObserver(static_cast<Observer*>(this));
 }
 
 } // namespace blink
 
 #endif // LifecycleObserver_h