Fix race at blink::GCInfoAtBase<T>::index

Source/platform/heap/Visitor.h

 /*
  * Copyright (C) 2013 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 370 matching lines @@
 
 private:
     static Visitor* fromHelper(VisitorHelper<Visitor>* helper) { return static_cast<Visitor*>(helper); }
 
     const MarkingMode m_markingMode;
     bool m_isGlobalMarkingVisitor;
 };
 
 template<typename T>
 struct TypenameStringTrait {
+    // This method is not thread safe.
     static const String& get()
     {
         DEFINE_STATIC_LOCAL(String, typenameString, (WTF::extractTypeNameFromFunctionName(WTF::extractNameFunction<T>())));
         return typenameString;
     }
 };
 
 } // namespace blink
 
 #endif // Visitor_h