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

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

Index: chrome/browser/resources/settings/site_settings/usb_devices.js
diff --git a/chrome/browser/resources/settings/site_settings/usb_devices.js b/chrome/browser/resources/settings/site_settings/usb_devices.js
index f0325621b1001975a4e4f47906090383b8cff8df..a80fc6f21cfee833e8c25f7017f47c7622b517b1 100644
--- a/chrome/browser/resources/settings/site_settings/usb_devices.js
+++ b/chrome/browser/resources/settings/site_settings/usb_devices.js
@@ -21,7 +21,7 @@ Polymer({
     devices_: Array,
 
     /**
-     * The targetted object for menu operations.
+     * The targeted object for menu operations.
      * @private {?Object}
      */
     actionMenuModel_: Object
@@ -43,6 +43,14 @@ Polymer({
   },
 
   /**
+   * @return {boolean}
+   * @private
+   */
+  hasDevices_: function() {
+    return !!(this.devices_ && this.devices_.length);
+  },
+
+  /**
    * A handler when an action is selected in the action menu.
    * @private
    */