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

Source/core/svg/properties/SVGListPropertyTearOffHelper.h

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 179 matching lines @@
             return nullptr;
         }
 
         RefPtr<ItemPropertyType> value = toDerived()->target()->appendItem(getValueForInsertionFromTearOff(item));
         toDerived()->commitChange();
 
         return createItemTearOff(value.release());
     }
 
 protected:
-    SVGListPropertyTearOffHelper(PassRefPtr<ListPropertyType> target, SVGElement* contextElement, PropertyIsAnimValType propertyIsAnimVal, const QualifiedName& attributeName = nullQName())
+    SVGListPropertyTearOffHelper(PassRefPtr<ListPropertyType> target, SVGElement* contextElement, PropertyIsAnimValType propertyIsAnimVal, const QualifiedName& attributeName = QualifiedName::null())
         : SVGPropertyTearOff<ListPropertyType>(target, contextElement, propertyIsAnimVal, attributeName)
     {
     }
 
     static PassRefPtr<ItemPropertyType> getValueForInsertionFromTearOff(PassRefPtr<ItemTearOffType> passNewItem)
     {
         return ItemTraits::getValueForInsertionFromTearOff(passNewItem);
     }
 
     PassRefPtr<ItemTearOffType> createItemTearOff(PassRefPtr<ItemPropertyType> value)
     {
         if (!value)
             return nullptr;
 
         return ItemTraits::createTearOff(value, toDerived()->contextElement(), toDerived()->propertyIsAnimVal(), toDerived()->attributeName());
     }
 
 private:
     Derived* toDerived() { return static_cast<Derived*>(this); }
 };
 
 }
 
 #endif // SVGListPropertyTearOffHelper_h