Supplement should have a Member to the corresponding Supplementable object (Part 1)

third_party/WebKit/Source/platform/Supplementable.h

 /*
  * Copyright (C) 2012 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
@@ skipping 75 matching lines @@
 //
 // Note that reattachThread() does nothing if assertion is not enabled.
 //
 
 template<typename T>
 class Supplementable;
 
 template<typename T>
 class Supplement : public GarbageCollectedMixin {
 public:
+    // TODO(haraken): Remove the default constructor.
+    // All Supplement objects should be instantiated with m_host.
+    Supplement() { }
+    explicit Supplement(T& host) : m_host(&host) { }
+    T* host() const { return m_host; }
+
     static void provideTo(Supplementable<T>& host, const char* key, Supplement<T>* supplement)
     {
         host.provideSupplement(key, supplement);
     }
 
     static Supplement<T>* from(Supplementable<T>& host, const char* key)
     {
         return host.requireSupplement(key);
     }
 
     static Supplement<T>* from(Supplementable<T>* host, const char* key)
     {
         return host ? host->requireSupplement(key) : 0;
     }
 
-    DEFINE_INLINE_VIRTUAL_TRACE() { }
+    DEFINE_INLINE_VIRTUAL_TRACE()
+    {
+        visitor->trace(m_host);
+    }
+
+private:
+    Member<T> m_host;
 };
 
 // Supplementable<T> inherits from GarbageCollectedMixin virtually
 // to allow ExecutionContext to derive from two GC mixin classes.
 template<typename T>
 class Supplementable : public virtual GarbageCollectedMixin {
     WTF_MAKE_NONCOPYABLE(Supplementable);
 public:
     void provideSupplement(const char* key, Supplement<T>* supplement)
     {
@@ skipping 49 matching lines @@
 };
 
 template<typename T>
 struct ThreadingTrait<Supplementable<T>> {
     static const ThreadAffinity Affinity = ThreadingTrait<T>::Affinity;
 };
 
 } // namespace blink
 
 #endif // Supplementable_h