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

Source/core/svg/SVGSVGElement.cpp

 /*
  * Copyright (C) 2004, 2005, 2006 Nikolas Zimmermann <zimmermann@kde.org>
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2010 Rob Buis <buis@kde.org>
  * Copyright (C) 2007 Apple Inc. All rights reserved.
  * Copyright (C) 2014 Google, Inc.
  *
  * 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 43 matching lines @@
 #include "platform/FloatConversion.h"
 #include "platform/LengthFunctions.h"
 #include "platform/geometry/FloatRect.h"
 #include "platform/transforms/AffineTransform.h"
 #include "wtf/StdLibExtras.h"
 
 namespace blink {
 
 inline SVGSVGElement::SVGSVGElement(Document& doc)
     : SVGGraphicsElement(SVGNames::svgTag, doc)
-    , SVGFitToViewBox(this)
     , m_x(SVGAnimatedLength::create(this, SVGNames::xAttr, SVGLength::create(LengthModeWidth), AllowNegativeLengths))
     , m_y(SVGAnimatedLength::create(this, SVGNames::yAttr, SVGLength::create(LengthModeHeight), AllowNegativeLengths))
     , m_width(SVGAnimatedLength::create(this, SVGNames::widthAttr, SVGLength::create(LengthModeWidth), ForbidNegativeLengths))
     , m_height(SVGAnimatedLength::create(this, SVGNames::heightAttr, SVGLength::create(LengthModeHeight), ForbidNegativeLengths))
     , m_useCurrentView(false)
     , m_timeContainer(SMILTimeContainer::create(*this))
     , m_translation(SVGPoint::create())
 {
+    SVGFitToViewBox::initialize(this);
+
     m_width->setDefaultValueAsString("100%");
     m_height->setDefaultValueAsString("100%");
 
     addToPropertyMap(m_x);
     addToPropertyMap(m_y);
     addToPropertyMap(m_width);
     addToPropertyMap(m_height);
 
     UseCounter::count(doc, UseCounter::SVGSVGElement);
 }
@@ skipping 680 matching lines @@
     visitor->trace(m_width);
     visitor->trace(m_height);
     visitor->trace(m_translation);
     visitor->trace(m_timeContainer);
     visitor->trace(m_viewSpec);
     SVGGraphicsElement::trace(visitor);
     SVGFitToViewBox::trace(visitor);
 }
 
 } // namespace blink