Bug 243621 Standardize QueryInterface without throw

r=neil sr=darin

suite/common/contentAreaUtils.js

@@ -441,12 +441,13 @@ nsHeaderSniffer.prototype = {
 
   // ---------- nsISupports methods ----------
   QueryInterface: function (iid) {
-    if (!iid.equals(Components.interfaces.nsIRequestObserver) &&
-        !iid.equals(Components.interfaces.nsISupports) &&
-        !iid.equals(Components.interfaces.nsIInterfaceRequestor)) {
-      throw Components.results.NS_ERROR_NO_INTERFACE;
-    }
+    if (iid.equals(Components.interfaces.nsIRequestObserver) ||
+        iid.equals(Components.interfaces.nsISupports) ||
+        iid.equals(Components.interfaces.nsIInterfaceRequestor))
       return this;
+
+    Components.returnCode = Components.results.NS_ERROR_NO_INTERFACE;
+    return null;
   },
 
   // ---------- nsIInterfaceRequestor methods ----------