4th patch for bug 158080

Description: replace the hardcode of "@mozilla.org/embedcomp/window-watcher;1" with NS_WINDOWWATCHER_CONTRACTID
Patch by Henry.Jia@sun.com
r=anto, sr=alecf

content/base/src/nsXMLHttpRequest.cpp

@@ -1503,7 +1503,7 @@ nsXMLHttpRequest::GetInterface(const nsIID & aIID, void **aResult)
     *aResult = nsnull;
 
     nsresult rv;
-    nsCOMPtr<nsIWindowWatcher> ww(do_GetService("@mozilla.org/embedcomp/window-watcher;1", &rv));
+    nsCOMPtr<nsIWindowWatcher> ww(do_GetService(NS_WINDOWWATCHER_CONTRACTID, &rv));
     if (NS_FAILED(rv))
       return rv;