cr-network-select: implement focus()

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-flex-layout/classes/iron-flex-layout.html">
 <link rel="import" href="chrome://resources/cr_elements/network/cr_network_list.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-flex;
       }
     </style>
-    <cr-network-list class ="flex" on-selected="onNetworkListItemSelected_"
+    <cr-network-list id="networkList" class ="flex"
+        on-selected="onNetworkListItemSelected_"
         on-network-connected="onNetworkConnected_"
         networks="[[networkStateList_]]" custom-items="[[customItems]]"
         show-buttons="[[showButtons]]"
         no-bottom-scroll-border="[[noBottomScrollBorder]]">
     </cr-network-list>
   </template>
   <script src="cr_network_select.js"></script>
 </dom-module>