Adds metronome time sync dbus client

chromeos/dbus/metronome_client.h

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef CHROMEOS_DBUS_METRONOME_CLIENT_H_
+#define CHROMEOS_DBUS_METRONOME_CLIENT_H_
+
+#include "base/basictypes.h"
+#include "base/callback_forward.h"
+#include "chromeos/dbus/dbus_client.h"
+#include "chromeos/dbus/dbus_client_implementation_type.h"
+
+namespace chromeos {
+
+// A DBus client class for the org.chromium.Metronome service.
+class MetronomeClient : public DBusClient {
+ public:
+  // Interface for observing timestamp events from a metronome client.
+  class Observer {
+   public:
+    // Called when the TimestampUpdated signal is received.
+    virtual void OnTimestampUpdated(uint64 beacon_timestamp,
+                                    uint64 mac_timestamp) = 0;

[Ben Chan, 2015/02/19 19:03:14]

|beacon_timestamp| and |mac_timestamp| kinda refers to similar thing.

|mac_timestamp| should be renamed to something else like |local_timestamp|

[varkha, 2015/02/19 20:33:43]

Done.

+  };
+
+  virtual ~MetronomeClient();
+
+  // Adds and removes observers for timestamp events.
+  virtual void AddObserver(Observer* observer) = 0;
+  virtual void RemoveObserver(Observer* observer) = 0;
+
+  // Factory function, creates a new instance and returns ownership.
+  // For normal usage, access the singleton via DBusThreadManager::Get().
+  static MetronomeClient* Create(DBusClientImplementationType type);
+
+ protected:
+  // Create() should be used instead.
+  MetronomeClient();
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(MetronomeClient);
+};
+
+}  // namespace chromeos
+
+#endif  // CHROMEOS_DBUS_METRONOME_CLIENT_H_