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

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

Index: ui/webui/resources/cr_elements/network/cr_network_select.js
diff --git a/ui/webui/resources/cr_elements/network/cr_network_select.js b/ui/webui/resources/cr_elements/network/cr_network_select.js
index eb9e50a24016c9cc34d7af1510d8afca3466b223..de4cdefe5b704bd52dbeaedda38321af396de824 100644
--- a/ui/webui/resources/cr_elements/network/cr_network_select.js
+++ b/ui/webui/resources/cr_elements/network/cr_network_select.js
@@ -10,6 +10,8 @@
 Polymer({
   is: 'cr-network-select',
 
+  behaviors: [I18nBehavior],
+
   properties: {
     /**
      * Network state for the active network.
@@ -198,4 +200,8 @@ Polymer({
         console.error('networkingPrivate.startConnect error: ' + lastError);
     });
   },
+
+  getExpandA11yText_: function() {
+    return this.i18n('networkExpandA11yLabel');
+  },
 });