[HBD] Update MD SiteSettingsHandler to support 3 modes.

chrome/test/data/webui/settings/test_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.
 
 /**
  * An example empty pref.
  * @type {SiteSettingsPref}
  */
 var prefsEmpty = {
   defaults: {
@@ skipping 109 matching lines @@
     } else if (contentType == settings.ContentSettingsTypes.PLUGINS) {
       pref = this.prefs_.defaults.plugins;
     } else if (
         contentType == settings.ContentSettingsTypes.UNSANDBOXED_PLUGINS) {
       pref = this.prefs_.defaults.unsandboxed_plugins;
     } else {
       console.log('getDefault received unknown category: ' + contentType);
     }
 
     assert(pref != undefined, 'Pref is missing for ' + contentType);
-    return Promise.resolve(pref != 'block');
+    return Promise.resolve(pref);
   },
 
   /** @override */
   getExceptionList: function(contentType) {
     this.methodCalled('getExceptionList', contentType);
 
     var pref = undefined;
     if (contentType == settings.ContentSettingsTypes.AUTOMATIC_DOWNLOADS)
       pref = this.prefs_.exceptions.auto_downloads;
     else if (contentType == settings.ContentSettingsTypes.BACKGROUND_SYNC)
@@ skipping 38 matching lines @@
   },
 
   /** @override */
   setCategoryPermissionForOrigin: function(
       primaryPattern, secondaryPattern, contentType, value) {
     this.methodCalled('setCategoryPermissionForOrigin',
         [primaryPattern, secondaryPattern, contentType, value]);
     return Promise.resolve();
   },
 };