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

Source/core/svg/SVGFitToViewBox.h

Index: Source/core/svg/SVGFitToViewBox.h
diff --git a/Source/core/svg/SVGFitToViewBox.h b/Source/core/svg/SVGFitToViewBox.h
index 1e0daef3debc526f1f80bc4fb5fc9e3400e466f6..acf3076f049da66e2d30641e3295ea108744c276 100644
--- a/Source/core/svg/SVGFitToViewBox.h
+++ b/Source/core/svg/SVGFitToViewBox.h
@@ -73,7 +73,10 @@ public:
     virtual void trace(Visitor*);
 
 protected:
-    explicit SVGFitToViewBox(SVGElement*, PropertyMapPolicy = PropertyMapPolicyAdd);
+    SVGFitToViewBox();
+
+    void initialize(SVGElement* contextElement, PropertyMapPolicy = PropertyMapPolicyAdd);
+
     void updateViewBox(const FloatRect&);
     void clearViewBox() { m_viewBox = nullptr; }
     void clearPreserveAspectRatio() { m_preserveAspectRatio = nullptr; }