Add desktop engagement metrics.

chrome/browser/metrics/desktop_engagement/desktop_engagement_service.h

+// 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.
+
+#ifndef CHROME_BROWSER_METRICS_DESKTOP_ENGAGEMENT_DESKTOP_ENGAGEMENT_SERVICE_H_
+#define CHROME_BROWSER_METRICS_DESKTOP_ENGAGEMENT_DESKTOP_ENGAGEMENT_SERVICE_H_
+
+#include <memory>
+#include <vector>
+
+#include "base/macros.h"
+#include "base/memory/weak_ptr.h"
+#include "base/time/time.h"
+#include "base/timer/timer.h"
+
+// Class for tracking and recording user engagement based on browser visibility,
+// audio and user interaction.
+class DesktopEngagementService {
+ public:
+  static void Initialize();

[Alexei Svitkine (slow), 2016/07/26 19:20:37]

Comment on all the functions please and the member variables.

[gayane -on leave until 09-2017, 2016/07/26 21:41:19]

Done.

+  static bool IsInitialized();
+  static DesktopEngagementService* Get();
+
+  void OnUserEvent();
+
+  void OnVisibilityChanged(bool visible);
+
+  void OnAudioStart();
+  void OnAudioEnd();
+
+ private:
+  friend class MockDesktopEngagementService;
+
+  DesktopEngagementService();
+  ~DesktopEngagementService();
+
+  virtual void OnTimerFired();
+  void StartTimer(base::TimeDelta duration);
+
+  void StartSession();
+  void EndSession();
+
+  void InitInactivityTimeout();
+
+  base::TimeTicks session_start_;
+  base::TimeTicks last_user_event_;
+
+  bool is_visible_ = false;
+  bool in_session_ = false;
+  bool is_audio_playing_ = false;
+  bool is_first_session_ = true;
+  base::TimeDelta inactivity_timeout_;
+
+  base::OneShotTimer timer_;
+
+  base::WeakPtrFactory<DesktopEngagementService> weak_factory_;
+
+  DISALLOW_COPY_AND_ASSIGN(DesktopEngagementService);
+};
+
+#endif  // CHROME_BROWSER_METRICS_DESKTOP_ENGAGEMENT_DESKTOP_ENGAGEMENT_SERVICE_H_