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

Source/core/svg/SVGAElement.cpp

 /*
  * Copyright (C) 2004, 2005, 2008 Nikolas Zimmermann <zimmermann@kde.org>
  * Copyright (C) 2004, 2005, 2007 Rob Buis <buis@kde.org>
  * Copyright (C) 2007 Eric Seidel <eric@webkit.org>
  * Copyright (C) 2010 Apple Inc. All rights reserved.
  *
  * 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.
@@ skipping 37 matching lines @@
 #include "core/svg/animation/SVGSMILElement.h"
 #include "platform/PlatformMouseEvent.h"
 #include "platform/network/ResourceRequest.h"
 
 namespace blink {
 
 using namespace HTMLNames;
 
 inline SVGAElement::SVGAElement(Document& document)
     : SVGGraphicsElement(SVGNames::aTag, document)
-    , SVGURIReference(this)
     , m_svgTarget(SVGAnimatedString::create(this, SVGNames::targetAttr, SVGString::create()))
     , m_wasFocusedByMouse(false)
 {
+    SVGURIReference::initialize(this);
+
     addToPropertyMap(m_svgTarget);
 }
 
 void SVGAElement::trace(Visitor* visitor)
 {
     visitor->trace(m_svgTarget);
     SVGGraphicsElement::trace(visitor);
     SVGURIReference::trace(visitor);
 }
 
@@ skipping 138 matching lines @@
         return SVGElement::canStartSelection();
     return hasEditableStyle();
 }
 
 bool SVGAElement::willRespondToMouseClickEvents()
 {
     return isLink() || SVGGraphicsElement::willRespondToMouseClickEvents();
 }
 
 } // namespace blink