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

Source/core/svg/SVGViewElement.cpp

Index: Source/core/svg/SVGViewElement.cpp
diff --git a/Source/core/svg/SVGViewElement.cpp b/Source/core/svg/SVGViewElement.cpp
index 54608fce4587e313b8e6ff5686744a3bde41ac43..e13be2c2ccaa9697a81fc1758f0d83f043e8cd12 100644
--- a/Source/core/svg/SVGViewElement.cpp
+++ b/Source/core/svg/SVGViewElement.cpp
@@ -26,9 +26,10 @@ namespace blink {
 
 inline SVGViewElement::SVGViewElement(Document& document)
     : SVGElement(SVGNames::viewTag, document)
-    , SVGFitToViewBox(this)
     , m_viewTarget(SVGStaticStringList::create(this, SVGNames::viewTargetAttr))
 {
+    SVGFitToViewBox::initialize(this);
+
     addToPropertyMap(m_viewTarget);
 }