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

Source/core/svg/SVGLengthListTearOff.h

Index: Source/core/svg/SVGLengthListTearOff.h
diff --git a/Source/core/svg/SVGLengthListTearOff.h b/Source/core/svg/SVGLengthListTearOff.h
index c3989f63bcbe49eabd0632132abdbfa9f7f9a92c..d18532c36ca431e2f17d5c3eb07b2412ac9e587b 100644
--- a/Source/core/svg/SVGLengthListTearOff.h
+++ b/Source/core/svg/SVGLengthListTearOff.h
@@ -40,13 +40,13 @@ class SVGLengthListTearOff FINAL :
     public SVGListPropertyTearOffHelper<SVGLengthListTearOff, SVGLengthList>,
     public ScriptWrappable {
 public:
-    static PassRefPtr<SVGLengthListTearOff> create(PassRefPtr<SVGLengthList> target, SVGElement* contextElement, PropertyIsAnimValType propertyIsAnimVal, const QualifiedName& attributeName = nullQName())
+    static PassRefPtr<SVGLengthListTearOff> create(PassRefPtr<SVGLengthList> target, SVGElement* contextElement, PropertyIsAnimValType propertyIsAnimVal, const QualifiedName& attributeName = QualifiedName::null())
     {
         return adoptRef(new SVGLengthListTearOff(target, contextElement, propertyIsAnimVal, attributeName));
     }
 
 private:
-    SVGLengthListTearOff(PassRefPtr<SVGLengthList> target, SVGElement* contextElement, PropertyIsAnimValType propertyIsAnimVal, const QualifiedName& attributeName = nullQName())
+    SVGLengthListTearOff(PassRefPtr<SVGLengthList> target, SVGElement* contextElement, PropertyIsAnimValType propertyIsAnimVal, const QualifiedName& attributeName = QualifiedName::null())
         : SVGListPropertyTearOffHelper<SVGLengthListTearOff, SVGLengthList>(target, contextElement, propertyIsAnimVal, attributeName)
     {
         ScriptWrappable::init(this);