Add desktop engagement metrics.

chrome/browser/metrics/desktop_engagement/chrome_visibility_observer.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_CHROME_VISIBILITY_OBSERVER_H_
+#define CHROME_BROWSER_METRICS_DESKTOP_ENGAGEMENT_CHROME_VISIBILITY_OBSERVER_H_
+
+#include "base/memory/weak_ptr.h"
+#include "chrome/browser/ui/browser_list_observer.h"
+
+namespace metrics {
+
+// Observer for tracking browser visibility events.
+class ChromeVisibilityObserver : public chrome::BrowserListObserver {
+ public:
+  ChromeVisibilityObserver();
+  ~ChromeVisibilityObserver() override;
+
+  // Initialize |ChromeVisibilityObserver| instance which observes |BrowserList|
+  // for browser visibility changes.
+  static void Initialize();
+
+ private:
+  // Notifies |DesktopEngagementService| of visibility changes. Overridden by
+  // tests.
+  virtual void SendVisibilityChangeEvent(bool active);

[Alexei Svitkine (slow), 2016/07/26 22:00:40]

I think having virtual functions that are private is confusing, even though it
does work in C++. I suggest making them protected instead.

[gayane -on leave until 09-2017, 2016/07/27 21:10:24]

Done.

+  // Cancels visibility change in case when the browser becomes visible after a
+  // short gap.
+  void CancelVisibilityChange();
+
+  // chrome::BrowserListObserver:
+  void OnBrowserSetLastActive(Browser* browser) override;
+  void OnBrowserNoLongerActive(Browser* browser) override;
+  void OnBrowserRemoved(Browser* browser) override;
+
+  // Sets |visibility_gap_timeout_| based on variation params.
+  void InitVisibilityGapTimeout();
+
+  // Timeout interval for waiting after loss of visibility. This allows merging
+  // two visibility session if they happened very shortly after each other, for
+  // example, when user switching between two browser windows.
+  base::TimeDelta visibility_gap_timeout_;
+
+  base::WeakPtrFactory<ChromeVisibilityObserver> weak_factory_;
+
+  DISALLOW_COPY_AND_ASSIGN(ChromeVisibilityObserver);
+};
+
+}  // namespace metrics
+
+#endif  // CHROME_BROWSER_METRICS_DESKTOP_ENGAGEMENT_CHROME_VISIBILITY_OBSERVER_H_