system_api: dbus: add metrics_event protobuffer

This is used for signaling memory pressure events from Chrome to
memd (or other interested party).

BUG=chromium:729335
TEST=compiles and runs correctly with chromium changes that use it

Change-Id: I00aee977045977d606e3f09eb8a4215c54ea5ee2
Reviewed-on: https://chromium-review.googlesource.com/1186024
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/metrics_event/service.proto b/dbus/metrics_event/service.proto
new file mode 100644
index 0000000..48fd473
--- /dev/null
+++ b/dbus/metrics_event/service.proto
@@ -0,0 +1,25 @@
+// Copyright 2018 The Chromium OS Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+syntax = "proto3";
+
+option optimize_for = LITE_RUNTIME;
+
+package metrics_event;
+
+// Content of MetricsEvent signals emitted by Chrome.
+message Event {
+  // Event types.
+  enum Type {
+    TAB_DISCARD = 0;
+    OOM_KILL = 1;
+    TAB_SWITCH = 2;
+  }
+
+  // The type of the event being signaled.
+  Type type = 1;
+
+  // The time of the event (CLOCK_MONOTONIC in milliseconds).
+  int64 timestamp = 2;
+}