Fix timeout callback leaks

Make sure that we don't leak oneshot timeout handlers in main.js and
polkitAuthenticationAgent.js by making the callback return false.

https://bugzilla.gnome.org/show_bug.cgi?id=668087

js/ui/main.js

@@ -332,6 +332,7 @@ function _windowRemoved(workspace, window) {
             workspace._lastRemovedWindow = null;
             _queueCheckWorkspaces();
         }
+        return false;
     });
 }
 

js/ui/polkitAuthenticationAgent.js

@@ -393,6 +393,7 @@ const AuthenticationAgent = new Lang.Class({
                                  Lang.bind(this,
                                            function() {
                                                this._reallyCompleteRequest(wasDismissed);
+                                               return false;
                                            }));
         } else {
             this._reallyCompleteRequest(wasDismissed);