Bug 231892 - gPrintPreviewObs should not throw for QI to nsISupports.

r/sr=neil

xpfe/browser/resources/content/browser.js

@@ -195,8 +195,11 @@ var gPrintPreviewObs = {
 
     QueryInterface : function(iid)
     {
-     if (iid.equals(Components.interfaces.nsIObserver) || iid.equals(Components.interfaces.nsISupportsWeakReference))
+     if (iid.equals(Components.interfaces.nsIObserver) || 
+         iid.equals(Components.interfaces.nsISupportsWeakReference) || 
+         iid.equals(Components.interfaces.nsISupports)) {
       return this;
+     }
      
      throw Components.results.NS_NOINTERFACE;
     }