Support rollback with installerdata in mini_installer

chrome/installer/util/master_preferences_constants.cc

Index: chrome/installer/util/master_preferences_constants.cc
diff --git a/chrome/installer/util/master_preferences_constants.cc b/chrome/installer/util/master_preferences_constants.cc
index 657da75ffad0307af0def570dd604330b87b9cd9..27eeed89193d9c0de0aece0081f32ad77f410ac0 100644
--- a/chrome/installer/util/master_preferences_constants.cc
+++ b/chrome/installer/util/master_preferences_constants.cc
@@ -42,5 +42,7 @@ namespace master_preferences {
   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