buffet: Add RegistrationStatus::InvalidCredentials

If the device has been deleted from GCD then an oauth request will
return an error of invalid_grant. Make sure we handle this and set
the registration status to InvalidCredentials.

BUG=brillo:16
TEST=FEATURES=test emerge-${BOARD} buffet (I also tested this manually
by deregistering the device using
https://gcd.sandbox.google.com/clouddevices#) and
test_that <ipaddress> buffet_Registration buffet_BasicDBusAPI

Change-Id: I60aa68186d9c6f3a7ce812e4dddfda626ea2b42f
Reviewed-on: https://chromium-review.googlesource.com/254170
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Commit-Queue: Nathan Bullock <nathanbullock@google.com>
Tested-by: Nathan Bullock <nathanbullock@google.com>
diff --git a/buffet/registration_status.h b/buffet/registration_status.h
index fa2e2cb..eee0c81 100644
--- a/buffet/registration_status.h
+++ b/buffet/registration_status.h
@@ -16,6 +16,7 @@
   kUnregistered,  // We have no credentials.
   kRegistering,  // We've just been given credentials.
   kRegistered,  // We're registered and online.
+  kInvalidCredentials,  // Our registration has been revoked.
 };
 
 std::string StatusToString(RegistrationStatus status);