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

third_party/WebKit/Source/platform/Supplementable.h

Index: third_party/WebKit/Source/platform/Supplementable.h
diff --git a/third_party/WebKit/Source/platform/Supplementable.h b/third_party/WebKit/Source/platform/Supplementable.h
index 487873705cf0e96439468a31084aa8268612a03a..3079076866c6914bedacfdbacbb29346cfc9fe8f 100644
--- a/third_party/WebKit/Source/platform/Supplementable.h
+++ b/third_party/WebKit/Source/platform/Supplementable.h
@@ -26,6 +26,7 @@
 #ifndef Supplementable_h
 #define Supplementable_h
 
+#include "platform/PlatformExport.h"
 #include "platform/heap/Handle.h"
 #include "wtf/Assertions.h"
 #include "wtf/HashMap.h"
@@ -91,8 +92,14 @@ template<typename T>
 class Supplementable;
 
 template<typename T>
-class Supplement : public GarbageCollectedMixin {
+class PLATFORM_EXPORT 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);
@@ -108,7 +115,13 @@ public:
         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