buffet: Expose RegistrationStatus over DBus

This new property lets applications monitor Buffet's connection to
cloud services.

BUG=brillo:16
TEST=Unittests, buffet_Registration has been expanded appropriately.
CQ-DEPEND=CL:*199337
Change-Id: I30253e8199cb65068a74dd8b780a8ab0954bf9fa
Reviewed-on: https://chromium-review.googlesource.com/250011
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Tested-by: Christopher Wiley <wiley@chromium.org>
Commit-Queue: Christopher Wiley <wiley@chromium.org>
diff --git a/buffet/manager.h b/buffet/manager.h
index d7d71bf..1c891ec 100644
--- a/buffet/manager.h
+++ b/buffet/manager.h
@@ -75,6 +75,8 @@
   // Handles calls to org.chromium.Buffet.Manager.Test()
   std::string TestMethod(const std::string& message) override;
 
+  void OnRegistrationStatusChange(RegistrationStatus status);
+
   org::chromium::Buffet::ManagerAdaptor dbus_adaptor_{this};
   chromeos::dbus_utils::DBusObject dbus_object_;