Initialize GC mixin bases only when leftmost vtable has been initialized.

Source/core/svg/SVGTests.h

 /*
  * Copyright (C) 2004, 2005, 2008 Nikolas Zimmermann <zimmermann@kde.org>
  * Copyright (C) 2004, 2005, 2006, 2010 Rob Buis <buis@kde.org>
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
@@ skipping 31 matching lines @@
     bool isValid(Document&) const;
 
     bool parseAttribute(const QualifiedName&, const AtomicString&);
     bool isKnownAttribute(const QualifiedName&);
 
     void addSupportedAttributes(HashSet<QualifiedName>&);
 
     virtual void trace(Visitor*);
 
 protected:
-    explicit SVGTests(SVGElement* contextElement);
+    SVGTests();
+
+    void initialize(SVGElement*);
 
 private:
     RefPtrWillBeMember<SVGStaticStringList> m_requiredFeatures;
     RefPtrWillBeMember<SVGStaticStringList> m_requiredExtensions;
     RefPtrWillBeMember<SVGStaticStringList> m_systemLanguage;
 };
 
 } // namespace blink
 
 #endif // SVGTests_h