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

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

 // Copyright 2016 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.
 
 /**
  * @fileoverview
  * 'usb-devices' is the polymer element for showing the USB Devices category
  * under Site Settings.
  */
 
 Polymer({
   is: 'usb-devices',
 
   behaviors: [SiteSettingsBehavior],
 
   properties: {
     /**
      * A list of all USB devices.
      * @private {!Array<!UsbDeviceEntry>}
      */
     devices_: Array,
 
     /**
-     * The targetted object for menu operations.
+     * The targeted object for menu operations.
      * @private {?Object}
      */
     actionMenuModel_: Object
   },
 
   /** @override */
   ready: function() {
     this.fetchUsbDevices_();
   },
 
   /**
    * Fetch the list of USB devices and update the list.
    * @private
    */
   fetchUsbDevices_: function() {
     this.browserProxy.fetchUsbDevices().then(function(deviceList) {
       this.devices_ = deviceList;
     }.bind(this));
   },
 
   /**
+   * @return {boolean}
+   * @private
+   */
+  hasDevices_: function() {
+    return !!(this.devices_ && this.devices_.length);
+  },
+
+  /**
    * A handler when an action is selected in the action menu.
    * @private
    */
   onRemoveTap_: function() {
     this.$$('dialog[is=cr-action-menu]').close();
 
     var item = this.actionMenuModel_;
     this.browserProxy.removeUsbDevice(
         item.origin, item.embeddingOrigin, item.object);
     this.actionMenuModel_ = null;
     this.fetchUsbDevices_();
   },
 
   /**
    * A handler to show the action menu next to the clicked menu button.
    * @param {!{model: !{item: UsbDeviceEntry}}} event
    * @private
    */
   showMenu_: function(event) {
     this.actionMenuModel_ = event.model.item;
     /** @type {!CrActionMenuElement} */ (
         this.$$('dialog[is=cr-action-menu]')).showAt(
             /** @type {!Element} */ (
                 Polymer.dom(/** @type {!Event} */ (event)).localTarget));
   }
 });