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

Source/core/svg/SVGImageElement.cpp

Index: Source/core/svg/SVGImageElement.cpp
diff --git a/Source/core/svg/SVGImageElement.cpp b/Source/core/svg/SVGImageElement.cpp
index a4a61a1d3e88b720dfd3825f430be9977687919c..e4313786a3e0633ace4db4a1e2210ca7e76c39df 100644
--- a/Source/core/svg/SVGImageElement.cpp
+++ b/Source/core/svg/SVGImageElement.cpp
@@ -32,7 +32,6 @@ namespace blink {
 
 inline SVGImageElement::SVGImageElement(Document& document)
     : SVGGraphicsElement(SVGNames::imageTag, document)
-    , SVGURIReference(this)
     , m_x(SVGAnimatedLength::create(this, SVGNames::xAttr, SVGLength::create(LengthModeWidth), AllowNegativeLengths))
     , m_y(SVGAnimatedLength::create(this, SVGNames::yAttr, SVGLength::create(LengthModeHeight), AllowNegativeLengths))
     , m_width(SVGAnimatedLength::create(this, SVGNames::widthAttr, SVGLength::create(LengthModeWidth), ForbidNegativeLengths))
@@ -41,6 +40,8 @@ inline SVGImageElement::SVGImageElement(Document& document)
     , m_imageLoader(SVGImageLoader::create(this))
     , m_needsLoaderURIUpdate(true)
 {
+    SVGURIReference::initialize(this);
+
     addToPropertyMap(m_x);
     addToPropertyMap(m_y);
     addToPropertyMap(m_width);