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

Source/core/svg/SVGPatternElement.cpp

Index: Source/core/svg/SVGPatternElement.cpp
diff --git a/Source/core/svg/SVGPatternElement.cpp b/Source/core/svg/SVGPatternElement.cpp
index 3b4d2eace7277a207f690b92ebe5b95aa0adbd04..db02bd7aada34c3147265c2c309adefaf35a7817 100644
--- a/Source/core/svg/SVGPatternElement.cpp
+++ b/Source/core/svg/SVGPatternElement.cpp
@@ -33,9 +33,6 @@ namespace blink {
 
 inline SVGPatternElement::SVGPatternElement(Document& document)
     : SVGElement(SVGNames::patternTag, document)
-    , SVGURIReference(this)
-    , SVGTests(this)
-    , SVGFitToViewBox(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))
@@ -44,6 +41,10 @@ inline SVGPatternElement::SVGPatternElement(Document& document)
     , m_patternUnits(SVGAnimatedEnumeration<SVGUnitTypes::SVGUnitType>::create(this, SVGNames::patternUnitsAttr, SVGUnitTypes::SVG_UNIT_TYPE_OBJECTBOUNDINGBOX))
     , m_patternContentUnits(SVGAnimatedEnumeration<SVGUnitTypes::SVGUnitType>::create(this, SVGNames::patternContentUnitsAttr, SVGUnitTypes::SVG_UNIT_TYPE_USERSPACEONUSE))
 {
+    SVGURIReference::initialize(this);
+    SVGTests::initialize(this);
+    SVGFitToViewBox::initialize(this);
+
     addToPropertyMap(m_x);
     addToPropertyMap(m_y);
     addToPropertyMap(m_width);