[MD settings] explain when no usb devices are shown in site settings

chrome/browser/resources/settings/site_settings/usb_devices.html

 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-icon-button/paper-icon-button.html">
 <link rel="import" href="chrome://resources/cr_elements/cr_action_menu/cr_action_menu.html">
 
 <link rel="import" href="../i18n_setup.html">
 <link rel="import" href="../settings_shared_css.html">
 <link rel="import" href="site_settings_behavior.html">
 <link rel="import" href="site_settings_prefs_browser_proxy.html">
 
 <dom-module id="usb-devices">
   <template>
     <style include="settings-shared">
       :host {
         display: block;
       }
 
       .column-header {
         -webkit-margin-start: 20px;
         margin-bottom: 15px;
         margin-top: 15px;
       }
     </style>
 
+    <div class="settings-box" hidden$="[[hasDevices_(devices_)]]">
+      $i18n{noUsbDevicesFound}
+    </div>
     <template is="dom-repeat" items="[[devices_]]">
       <div class="column-header">[[item.objectName]]</div>
 
       <div class="list-frame menu-content vertical-list">
         <div class="list-item">
           <div class="favicon-image"
               style$="[[computeSiteIcon(item.origin)]]"></div>
           <div class="middle">[[item.origin]]</div>
 
           <paper-icon-button icon="cr:more-vert" on-tap="showMenu_"
               class="dropdown-trigger" title="$i18n{moreActions}">
           </paper-icon-button>
         </div>
       </div>
     </template>
 
     <dialog is="cr-action-menu">
       <button class="dropdown-item" role="option" on-tap="onRemoveTap_" id="removeButton">
         $i18n{handlerRemove}
       </button>
     </dialog>
 
   </template>
   <script src="usb_devices.js"></script>
 </dom-module>