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

Source/core/dom/QualifiedName.cpp

Index: Source/core/dom/QualifiedName.cpp
diff --git a/Source/core/dom/QualifiedName.cpp b/Source/core/dom/QualifiedName.cpp
index 4400201a5628d53885e0715f4127da8e0b7977e7..9ca243ee98576a5c5d059d47fcd05add60877eb2 100644
--- a/Source/core/dom/QualifiedName.cpp
+++ b/Source/core/dom/QualifiedName.cpp
@@ -21,8 +21,6 @@
 
 #ifdef SKIP_STATIC_CONSTRUCTORS_ON_GCC
 #define WEBCORE_QUALIFIEDNAME_HIDE_GLOBALS 1
-#else
-#define QNAME_DEFAULT_CONSTRUCTOR
 #endif
 
 #include "HTMLNames.h"
@@ -114,6 +112,8 @@ const QualifiedName& nullQName()
 
 const AtomicString& QualifiedName::localNameUpper() const
 {
+    if (!m_impl)
+        return nullAtom;
     if (!m_impl->m_localNameUpper)
         m_impl->m_localNameUpper = m_impl->m_localName.upper();
     return m_impl->m_localNameUpper;