Bug 231893 - Allow QI to nsISupports.

r+sr=blake
p=116057@bacon.qcc.mass.edu (Justin Wood)

toolkit/components/printing/content/printPreviewProgress.js

@@ -102,7 +102,9 @@ var progressListener = {
 
   QueryInterface: function (iid)
   {
-    if (iid.equals(Components.interfaces.nsIWebProgressListener) || iid.equals(Components.interfaces.nsISupportsWeakReference))
+    if (iid.equals(Components.interfaces.nsIWebProgressListener) ||
+        iid.equals(Components.interfaces.nsISupportsWeakReference) ||
+        iid.equals(Components.interfaces.nsISupports))
       return this;   
     throw Components.results.NS_NOINTERFACE;
   }