[MD-Settings A11y] Set alt text for expandable buttons.

ui/webui/resources/cr_elements/network/cr_network_select.js

 // 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.
 
 /**
  * @fileoverview Polymer element wrapping cr-network-list including the
  * networkingPrivate calls to populate it.
  */
 
 Polymer({
   is: 'cr-network-select',
 
+  behaviors: [I18nBehavior],
+
   properties: {
     /**
      * Network state for the active network.
      * @type {?CrOnc.NetworkStateProperties}
      */
     activeNetworkState: Object,
 
     /**
      * If true, the element includes an 'expand' button that toggles the
      * expanded state of the network list.
@@ skipping 168 matching lines @@
 
     if (state.ConnectionState != CrOnc.ConnectionState.NOT_CONNECTED)
       return;
 
     chrome.networkingPrivate.startConnect(state.GUID, function() {
       var lastError = chrome.runtime.lastError;
       if (lastError && lastError != 'connecting')
         console.error('networkingPrivate.startConnect error: ' + lastError);
     });
   },
+
+  getExpandA11yText_: function() {
+    return this.i18n('networkExpandA11yLabel');
+  },
 });