Add update available icon in system tray

chrome/browser/upgrade_observer.h

+// Copyright 2017 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 CHROME_BROWSER_UPGRADE_OBSERVER_H_
+#define CHROME_BROWSER_UPGRADE_OBSERVER_H_
+
+#include "base/macros.h"
+
+class UpgradeObserver {
+ public:
+  // Triggered when a software update is available, but downloading requires
+  // user's agreement as current connection is cellular.
+  virtual void OnUpdateOverCellularAvailable() = 0;
+
+ protected:
+  virtual ~UpgradeObserver() {}
+};
+
+#endif  // CHROME_BROWSER_UPGRADE_OBSERVER_H_