system_api: service_constants.h: add MetricsEvent

This adds constants for the MetricsEventService.
Ideally this would have gone in with the addition
of metrics_event.proto, but I had my chrome and cros
sources out of sync.

BUG=chromium:729335
TEST=chromium compiles (cros clones these in Rust code)

Change-Id: I2407619196dd6fe75e3143caa932bb443ad5fffc
Reviewed-on: https://chromium-review.googlesource.com/1204910
Commit-Ready: Luigi Semenzato <semenzato@chromium.org>
Tested-by: Luigi Semenzato <semenzato@chromium.org>
Reviewed-by: Dan Erat <derat@chromium.org>
diff --git a/dbus/service_constants.h b/dbus/service_constants.h
index f828c4b..0a0218a 100644
--- a/dbus/service_constants.h
+++ b/dbus/service_constants.h
@@ -126,6 +126,12 @@
     "org.chromium.LivenessServiceInterface";
 const char kLivenessServiceCheckLivenessMethod[] = "CheckLiveness";
 
+const char kMetricsEventServiceName[] = "org.chromium.MetricsEventService";
+const char kMetricsEventServicePath[] = "/org/chromium/MetricsEventService";
+const char kMetricsEventServiceInterface[] =
+    "org.chromium.MetricsEventServiceInterface";
+const char kMetricsEventServiceChromeEventSignal[] = "ChromeEvent";
+
 const char kComponentUpdaterServiceName[] =
     "org.chromium.ComponentUpdaterService";
 const char kComponentUpdaterServicePath[] =