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

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

 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-collapse/iron-collapse.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-flex-layout/classes/iron-flex-layout.html">
 <link rel="import" href="chrome://resources/cr_elements/cr_expand_button/cr_expand_button.html">
 <link rel="import" href="chrome://resources/cr_elements/network/cr_network_list.html">
 <link rel="import" href="chrome://resources/cr_elements/network/cr_network_list_item.html">
 <link rel="import" href="chrome://resources/cr_elements/network/cr_onc_types.html">
 
 <dom-module id="cr-network-select">
   <template>
     <style>
       :host {
         display: inline-block;
       }
     </style>
     <div class="layout vertical">
       <div class="layout horizontal center">
         <cr-network-list-item class="flex" hidden$="[[!showActive]]"
             item="[[activeNetworkState]]">
         </cr-network-list-item>
         <cr-expand-button toggles hidden$="[[!expandable]]"
+            alt="[[getExpandA11yText_()]]"
             expanded="{{networkListOpened}}">
         </cr-expand-button>
       </div>
       <iron-collapse opened="[[networkListOpened]]">
         <cr-network-list on-selected="onNetworkListItemSelected_"
             max-height="[[maxHeight]]" networks="[[networkStateList]]"
             custom-items="[[customItems]]" show-buttons=[[showButtons]]
             show-separators="[[showSeparators]]">
         </cr-network-list>
       </iron-collapse>
     </div>
   </template>
   <script src="cr_network_select.js"></script>
 </dom-module>