[oilpan]: Avoid refcounting QualifiedName's nullQName when tracing.

Source/core/svg/SVGStringListTearOff.h

Index: Source/core/svg/SVGStringListTearOff.h
diff --git a/Source/core/svg/SVGStringListTearOff.h b/Source/core/svg/SVGStringListTearOff.h
index 084bc4dfa188849d7b1fe70e8facab4c28ee87e9..c012d3d49210245360fa076fee432cb476fd7350 100644
--- a/Source/core/svg/SVGStringListTearOff.h
+++ b/Source/core/svg/SVGStringListTearOff.h
@@ -38,7 +38,7 @@ namespace WebCore {
 
 class SVGStringListTearOff : public SVGPropertyTearOff<SVGStringList>, public ScriptWrappable {
 public:
-    static PassRefPtr<SVGStringListTearOff> create(PassRefPtr<SVGStringList> target, SVGElement* contextElement, PropertyIsAnimValType propertyIsAnimVal, const QualifiedName& attributeName = nullQName())
+    static PassRefPtr<SVGStringListTearOff> create(PassRefPtr<SVGStringList> target, SVGElement* contextElement, PropertyIsAnimValType propertyIsAnimVal, const QualifiedName& attributeName = QualifiedName::null())
     {
         return adoptRef(new SVGStringListTearOff(target, contextElement, propertyIsAnimVal, attributeName));
     }