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_service.h b/dbus_service.h
index e99caef..6d449df 100644
--- a/dbus_service.h
+++ b/dbus_service.h
@@ -51,6 +51,13 @@
                                               gchar* omaha_url,
                                               GError **error);
 
+gboolean update_engine_service_attempt_update_with_flags(
+    UpdateEngineService* self,
+    gchar* app_version,
+    gchar* omaha_url,
+    gint flags_as_int,
+    GError **error);
+
 gboolean update_engine_service_attempt_rollback(UpdateEngineService* self,
                                                 gboolean powerwash,
                                                 GError **error);