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

Source/core/svg/SVGGraphicsElement.cpp

Index: Source/core/svg/SVGGraphicsElement.cpp
diff --git a/Source/core/svg/SVGGraphicsElement.cpp b/Source/core/svg/SVGGraphicsElement.cpp
index 6772b8d1781509dd4335843b5d8aa1ef15d2ad30..23cbbff77ff647ddb56e98ded9735d25855f9f8f 100644
--- a/Source/core/svg/SVGGraphicsElement.cpp
+++ b/Source/core/svg/SVGGraphicsElement.cpp
@@ -34,9 +34,10 @@ namespace blink {
 
 SVGGraphicsElement::SVGGraphicsElement(const QualifiedName& tagName, Document& document, ConstructionType constructionType)
     : SVGElement(tagName, document, constructionType)
-    , SVGTests(this)
     , m_transform(SVGAnimatedTransformList::create(this, SVGNames::transformAttr, SVGTransformList::create()))
 {
+    SVGTests::initialize(this);
+
     addToPropertyMap(m_transform);
 }