mash: Introduce WmSystemTrayNotifier for //ash/common

ash/common/system/tray/wm_system_tray_notifier.cc

+// Copyright 2016 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.
+
+#include "ash/common/system/tray/wm_system_tray_notifier.h"
+
+#include "ash/common/system/update/update_observer.h"
+
+namespace ash {
+
+WmSystemTrayNotifier::WmSystemTrayNotifier() {}
+
+WmSystemTrayNotifier::~WmSystemTrayNotifier() {}
+
+void WmSystemTrayNotifier::AddUpdateObserver(UpdateObserver* observer) {
+  update_observers_.AddObserver(observer);
+}
+
+void WmSystemTrayNotifier::RemoveUpdateObserver(UpdateObserver* observer) {
+  update_observers_.RemoveObserver(observer);
+}
+
+void WmSystemTrayNotifier::NotifyUpdateRecommended(const UpdateInfo& info) {
+  FOR_EACH_OBSERVER(UpdateObserver, update_observers_,
+                    OnUpdateRecommended(info));
+}
+
+}  // namespace ash