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

Source/core/svg/SVGMPathElement.cpp

 /*
  * Copyright (C) 2007 Eric Seidel <eric@webkit.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,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 13 matching lines @@
 #include "core/XLinkNames.h"
 #include "core/dom/Document.h"
 #include "core/svg/SVGAnimateMotionElement.h"
 #include "core/svg/SVGDocumentExtensions.h"
 #include "core/svg/SVGPathElement.h"
 
 namespace blink {
 
 inline SVGMPathElement::SVGMPathElement(Document& document)
     : SVGElement(SVGNames::mpathTag, document)
-    , SVGURIReference(this)
 {
+    SVGURIReference::initialize(this);
 }
 
 void SVGMPathElement::trace(Visitor* visitor)
 {
     SVGElement::trace(visitor);
     SVGURIReference::trace(visitor);
 }
 
 DEFINE_NODE_FACTORY(SVGMPathElement)
 
@@ skipping 78 matching lines @@
     notifyParentOfPathChange(parentNode());
 }
 
 void SVGMPathElement::notifyParentOfPathChange(ContainerNode* parent)
 {
     if (isSVGAnimateMotionElement(parent))
         toSVGAnimateMotionElement(parent)->updateAnimationPath();
 }
 
 } // namespace blink