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/buffet.gyp b/buffet/buffet.gyp
index 6628f4d..51312e5 100644
--- a/buffet/buffet.gyp
+++ b/buffet/buffet.gyp
@@ -37,6 +37,7 @@
         'dbus_bindings/org.chromium.Buffet.Manager.xml',
         'dbus_constants.cc',
         'manager.cc',
+        'registration_status.cc',
         'storage_impls.cc',
         'states/error_codes.cc',
         'states/state_change_queue.cc',