Make gSerialCounter defined only ifdef DEBUG to silence a warning. Followup to bug 416939.

dom/src/base/nsGlobalWindow.cpp

@@ -209,11 +209,14 @@ nsIFactory *nsGlobalWindow::sComputedDOMStyleFactory   = nsnull;
 
 static nsIEntropyCollector *gEntropyCollector          = nsnull;
 static PRInt32              gRefCnt                    = 0;
-static PRUint32             gSerialCounter             = 0;
 static PRInt32              gOpenPopupSpamCount        = 0;
 static PopupControlState    gPopupControlState         = openAbused;
 static PRInt32              gRunningTimeoutDepth       = 0;
 
+#ifdef DEBUG
+static PRUint32             gSerialCounter             = 0;
+#endif
+
 #ifdef DEBUG_jst
 PRInt32 gTimeoutCnt                                    = 0;
 #endif