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

Source/core/svg/animation/SVGSMILElement.cpp

Index: Source/core/svg/animation/SVGSMILElement.cpp
diff --git a/Source/core/svg/animation/SVGSMILElement.cpp b/Source/core/svg/animation/SVGSMILElement.cpp
index 39b1a75d41bc68b944eb0c5e9bc2e366608fb049..c52eac9d354eb11f41e3bbf7cc35e9d0104545fa 100644
--- a/Source/core/svg/animation/SVGSMILElement.cpp
+++ b/Source/core/svg/animation/SVGSMILElement.cpp
@@ -171,7 +171,6 @@ SVGSMILElement::Condition::Condition(Type type, BeginOrEnd beginOrEnd, const Str
 
 SVGSMILElement::SVGSMILElement(const QualifiedName& tagName, Document& doc)
     : SVGElement(tagName, doc)
-    , SVGTests(this)
     , m_attributeName(anyQName())
     , m_targetElement(nullptr)
     , m_syncBaseConditionsConnected(false)
@@ -190,6 +189,8 @@ SVGSMILElement::SVGSMILElement(const QualifiedName& tagName, Document& doc)
     , m_cachedMin(invalidCachedTime)
     , m_cachedMax(invalidCachedTime)
 {
+    SVGTests::initialize(this);
+
     resolveFirstInterval();
 }