Add unit test for lock and unlock virtual keyboard feature

ui/keyboard/resources/elements/kb-options-menu.html

 <!--
   -- Copyright 2013 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.
   -->
 
 <polymer-element name="kb-options-menu-item" attributes="layout label active"
     on-pointerup="up" on-pointerover="over" on-pointerout="out">
   <template>
     <style>
@@ skipping 114 matching lines @@
         }
       }
     </style>
     <!-- Insert popups here. -->
     <kb-options-menu id="options" hidden></kb-options-menu>
   </template>
   <script>
     Polymer('kb-keyboard-overlay', {
       up: function() {
         this.hidden = true;
+      },
+
+      hiddenChanged: function() {
+        this.fire('stateChange', {
+          state: 'overlayVisibility',
+          value: !!this.hidden
+        });
       }
     });
   </script>
 </polymer-element>