Support rollback with installerdata in mini_installer

chrome/installer/util/master_preferences_constants.cc

 // Copyright (c) 2012 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.
 
 #include "chrome/installer/util/master_preferences_constants.h"
 
 namespace installer {
 namespace master_preferences {
   const char kChrome[] = "chrome";
   const char kCreateAllShortcuts[] = "create_all_shortcuts";
@@ skipping 24 matching lines @@
   const char kMakeChromeDefaultForUser[] = "make_chrome_default_for_user";
   const char kSuppressFirstRunDefaultBrowserPrompt[] =
       "suppress_first_run_default_browser_prompt";
   const char kMsi[] = "msi";
   const char kMsiProductId[] = "msi_product_id";
   const char kMultiInstall[] = "multi_install";
   const char kRequireEula[] = "require_eula";
   const char kSystemLevel[] = "system_level";
   const char kVerboseLogging[] = "verbose_logging";
   const char kExtensionsBlock[] = "extensions.settings";
+  const char kAllowDowngrade[] = "allow_downgrade";
+
 }  // namespace master_preferences
 }  // namespace installer