Introduce infrastructure for tracing ScriptWrappables.

third_party/WebKit/Source/bindings/core/v8/ActiveScriptWrappable.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 ActiveScriptWrappable_h
 #define ActiveScriptWrappable_h
 
 #include "core/CoreExport.h"
 #include "wtf/Noncopyable.h"
 
 namespace blink {
 
 class ScriptWrappable;
+class ScriptWrappableVisitor;
 
 /**
  * Classes deriving from ActiveScriptWrappable will be registered in a
  * thread-specific list. They keep their wrappers and dependant objects alive
  * as long as they have pending activity.
  */
 class CORE_EXPORT ActiveScriptWrappable {
     WTF_MAKE_NONCOPYABLE(ActiveScriptWrappable);
 public:
     explicit ActiveScriptWrappable(ScriptWrappable*);
 
+    static void traceActiveScriptWrappables(ScriptWrappableVisitor*);
+
     virtual bool hasPendingActivity() const = 0;
 
     ScriptWrappable* toScriptWrappable() const;
 
 protected:
     virtual ~ActiveScriptWrappable();
 
 private:
     ScriptWrappable* m_scriptWrappable;
 };
 
 } // namespace blink
 
 #endif // ActiveScriptWrappable_h