update_engine_client: Add -interactive=true|false flag

This makes it easier to test the update_engine codepaths dealing with
non-interactive update attempts.

BUG=chromium:273251
TEST=Manually tested with 'update_engine_client -update -interactive=X' +
    unit tests pass.

Change-Id: Id3894261fd1c0dc2afdf2426484835f487924433
Reviewed-on: https://chromium-review.googlesource.com/168445
Reviewed-by: Chris Sosa <sosa@chromium.org>
Tested-by: David Zeuthen <zeuthen@chromium.org>
Commit-Queue: David Zeuthen <zeuthen@chromium.org>
diff --git a/dbus_constants.h b/dbus_constants.h
index e4173f0..625ac8d 100644
--- a/dbus_constants.h
+++ b/dbus_constants.h
@@ -12,6 +12,12 @@
     "/org/chromium/UpdateEngine";
 static const char* const kUpdateEngineServiceInterface =
     "org.chromium.UpdateEngineInterface";
+
+// Flags used in the AttemptUpdateWithFlags() D-Bus method.
+typedef enum {
+  kAttemptUpdateFlagNonInteractive = (1<<0)
+} AttemptUpdateFlags;
+
 }  // namespace chromeos_update_engine
 
 #endif  // CHROMEOS_PLATFORM_UPDATE_ENGINE_DBUS_CONSTANTS_H__