Support rollback with installerdata in mini_installer

chrome/installer/util/master_preferences_constants.h

Index: chrome/installer/util/master_preferences_constants.h
diff --git a/chrome/installer/util/master_preferences_constants.h b/chrome/installer/util/master_preferences_constants.h
index c2e1c9f49b8d8c327f7b977234bfbf963d86fe07..d49947bc2293ff52af73d055f80364836ad40061 100644
--- a/chrome/installer/util/master_preferences_constants.h
+++ b/chrome/installer/util/master_preferences_constants.h
@@ -92,6 +92,9 @@ extern const char kSystemLevel[];
 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