Support rollback with installerdata in mini_installer

chrome/installer/util/master_preferences_constants.h

 // 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.
 //
 // This file contains the constants used to process master_preferences files
 // used by setup and first run.
 
 #ifndef CHROME_INSTALLER_UTIL_MASTER_PREFERENCES_CONSTANTS_H_
 #define CHROME_INSTALLER_UTIL_MASTER_PREFERENCES_CONSTANTS_H_
 
@@ skipping 74 matching lines @@
 // shown. Relevant in Windows 8+ context only. If this is true, the standard
 // 'set default browser' prompt on the butter-bar will appear during the first
 // run.
 extern const char kSuppressFirstRunDefaultBrowserPrompt[];
 // Boolean. Install Chrome to system wise location. Cmd line override present.
 extern const char kSystemLevel[];
 // Boolean. Run installer in verbose mode. Cmd line override present.
 extern const char kVerboseLogging[];
 // Name of the block that contains the extensions on the master preferences.
 extern const char kExtensionsBlock[];
+// Boolean. Allow Chrome to be downgraded to a previous version if true.
+extern const char kAllowDowngrade[];
+
 }  // namespace master_preferences
 }  // namespace installer
 
 #endif  // CHROME_INSTALLER_UTIL_MASTER_PREFERENCES_CONSTANTS_H_