AU: Provide a reboot_if_needed D-Bus API.

This will reboot if the current state is UPDATED_REBOOT_NEEDED.
Also add -reboot option to the update_engine_client.

BUG=4250
TEST=unit tests,gmerged on device and tried reboot requests

Review URL: http://codereview.chromium.org/3034026
diff --git a/dbus_service.h b/dbus_service.h
index 4b760ce..12c9ed1 100644
--- a/dbus_service.h
+++ b/dbus_service.h
@@ -46,6 +46,14 @@
 
 // Methods
 
+gboolean update_engine_service_attempt_update(UpdateEngineService* self,
+                                              gchar* app_version,
+                                              gchar* omaha_url,
+                                              GError **error);
+
+gboolean update_engine_service_check_for_update(UpdateEngineService* self,
+                                                GError **error);
+
 gboolean update_engine_service_get_status(UpdateEngineService* self,
                                           int64_t* last_checked_time,
                                           double* progress,
@@ -54,14 +62,9 @@
                                           int64_t* new_size,
                                           GError **error);
 
-gboolean update_engine_service_check_for_update(UpdateEngineService* self,
+gboolean update_engine_service_reboot_if_needed(UpdateEngineService* self,
                                                 GError **error);
 
-gboolean update_engine_service_attempt_update(UpdateEngineService* self,
-                                              gchar* app_version,
-                                              gchar* omaha_url,
-                                              GError **error);
-
 gboolean update_engine_service_emit_status_update(
     UpdateEngineService* self,
     gint64 last_checked_time,