Mojo C++ bindings: support enum validation.

components/arc/common/net.mojom

 // 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.
 
 module arc;
 
+[Extensible=True]
 enum NetworkResult {
   SUCCESS = 0,
   FAILURE = 1,
 };
 
 struct WifiConfiguration {
   // These correspond to ONC properties returned by
   // chrome.networkingPrivate.getNetworks().
   // See components/onc/docs/onc_spec.html
   string ssid;
@@ skipping 11 matching lines @@
 interface NetHost {
   // Sends a request to get configured or visible WiFi networks based on the
   // |configured_only| and |visible_only| flags.
   GetNetworks@0(bool configured_only, bool visible_only) => (NetworkData data);
 };
 
 interface NetInstance {
   // Establishes full-duplex communication with the host.
   Init@0(NetHost host_ptr);
 };