Integrate audible tab tracking into desktop engagement service.

chrome/browser/metrics/chrome_visibility_observer.h

+// Copyright (c) 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_CHROME_VISIBILITY_OBSERVER_H_
+#define CHROME_BROWSER_METRICS_CHROME_VISIBILITY_OBSERVER_H_
+
+#include "base/memory/weak_ptr.h"
+#include "chrome/browser/ui/browser_list_observer.h"
+
+namespace metrics {
+
+class ChromeVisibilityObserver : public chrome::BrowserListObserver {
+ public:
+  ChromeVisibilityObserver();
+  ~ChromeVisibilityObserver() override;
+
+  static ChromeVisibilityObserver* GetInstance();
+
+  static void Initialize();
+
+ private:
+  void SendVisibilityChangeEvent(bool active);
+  void CancelVisibilityChange();
+
+  // chrome::BrowserListObserver:
+  void OnBrowserSetLastActive(Browser* browser) override;
+  void OnBrowserNoLongerActive(Browser* browser) override;
+  void OnBrowserAdded(Browser* browser) override;
+  void OnBrowserRemoved(Browser* browser) override;
+
+  int number_of_browsers_;
+  base::WeakPtrFactory<ChromeVisibilityObserver> weak_factory_;
+
+  DISALLOW_COPY_AND_ASSIGN(ChromeVisibilityObserver);
+};
+
+}  // namespace metrics
+
+#endif  // CHROME_BROWSER_METRICS_CHROME_VISIBILITY_OBSERVER_H_