[HBD] Update MD SiteSettingsHandler to support 3 modes.

chrome/browser/resources/settings/site_settings/site_settings_prefs_browser_proxy.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 A helper object used from the "Site Settings" section to
  * interact with the content settings prefs.
  */
 
 /**
@@ skipping 61 matching lines @@
     /**
      * Sets the default value for a site settings category.
      * @param {string} contentType The name of the category to change.
      * @param {string} defaultValue The name of the value to set as default.
      */
     setDefaultValueForContentType: function(contentType, defaultValue) {},
 
     /**
      * Gets the default value for a site settings category.
      * @param {string} contentType The name of the category to query.
-     * @return {Promise<boolean>}
+     * @return {Promise<string>} The string value of the default setting.
      */
     getDefaultValueForContentType: function(contentType) {},
 
     /**
      * Gets the exceptions (site list) for a particular category.
      * @param {string} contentType The name of the category to query.
      * @return {Promise<Array<SiteException>>}
      */
     getExceptionList: function(contentType) {},
 
@@ skipping 217 matching lines @@
     removeUsbDevice: function(origin, embeddingOrigin, usbDevice) {
       chrome.send('removeUsbDevice', [origin, embeddingOrigin, usbDevice]);
     },
   };
 
   return {
     SiteSettingsPrefsBrowserProxy: SiteSettingsPrefsBrowserProxy,
     SiteSettingsPrefsBrowserProxyImpl: SiteSettingsPrefsBrowserProxyImpl,
   };
 });