Add tests for the D-Bus MessageFilter.

Review URL: http://codereview.chromium.org/1990001
diff --git a/metrics/metrics_daemon.h b/metrics/metrics_daemon.h
index 028525a..3ab857a 100644
--- a/metrics/metrics_daemon.h
+++ b/metrics/metrics_daemon.h
@@ -39,6 +39,7 @@
   FRIEND_TEST(MetricsDaemonTest, LookupPowerState);
   FRIEND_TEST(MetricsDaemonTest, LookupScreenSaverState);
   FRIEND_TEST(MetricsDaemonTest, LookupSessionState);
+  FRIEND_TEST(MetricsDaemonTest, MessageFilter);
   FRIEND_TEST(MetricsDaemonTest, NetStateChanged);
   FRIEND_TEST(MetricsDaemonTest, PowerStateChanged);
   FRIEND_TEST(MetricsDaemonTest, PublishMetric);