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

Source/core/svg/SVGCursorElement.cpp

Index: Source/core/svg/SVGCursorElement.cpp
diff --git a/Source/core/svg/SVGCursorElement.cpp b/Source/core/svg/SVGCursorElement.cpp
index e0e606d52f1c64348f16ae317919cff1af5d1ef6..bf27f2c564b41a851194ff38768b15ae5e453b9b 100644
--- a/Source/core/svg/SVGCursorElement.cpp
+++ b/Source/core/svg/SVGCursorElement.cpp
@@ -29,11 +29,12 @@ namespace blink {
 
 inline SVGCursorElement::SVGCursorElement(Document& document)
     : SVGElement(SVGNames::cursorTag, document)
-    , SVGTests(this)
-    , SVGURIReference(this)
     , m_x(SVGAnimatedLength::create(this, SVGNames::xAttr, SVGLength::create(LengthModeWidth), AllowNegativeLengths))
     , m_y(SVGAnimatedLength::create(this, SVGNames::yAttr, SVGLength::create(LengthModeHeight), AllowNegativeLengths))
 {
+    SVGTests::initialize(this);
+    SVGURIReference::initialize(this);
+
     addToPropertyMap(m_x);
     addToPropertyMap(m_y);
 }