networking.config API: Add an example to the documentation.

extensions/common/api/networking_config.idl

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Use the <code>networking.config</code> API to authenticate to captive
 // portals.
 namespace networking.config {
   // Indicator for the type of network used in $(ref:NetworkInfo).
   enum NetworkType { WiFi };
 
@@ skipping 66 matching lines @@
 
     // Called by the extension to notify the network config API that it finished
     // a captive portal authentication attempt and hand over the result of the
     // attempt. This function must only be called with the GUID of the latest
     // $(ref:onCaptivePortalDetected) event.
     // |GUID|: Unique network identifier obtained from
     //         $(ref:onCaptivePortalDetected).
     // |result|: The result of the authentication attempt.
     // |callback|: Called back when this operation is finished.
     void finishAuthentication(DOMString GUID, AuthenticationResult result,
-                              FinishAuthenticationCallback callback);
+                              optional FinishAuthenticationCallback callback);
   };
 
   interface Events {
     // This event fires everytime a captive portal is detected on a network
     // matching any of the currently registered network filters and the user
     // consents to use the extension for authentication. Network filters may be
     // set using the $(ref:setNetworkFilter).
     // Upon receiving this event the extension should start its authentication
     // attempt with the captive portal. When the extension finishes its attempt,
     // it must call $(ref:finishAuthentication) with the <code>GUID</code>
     // received with this event and the appropriate authentication result.
     // |networkInfo|: Information about the network on which a captive portal
     // was detected.
     static void onCaptivePortalDetected(NetworkInfo networkInfo);
   };
 };