Introduce TabStripModelStatsRecorder class to record tab interaction stats

chrome/browser/ui/tabs/tab_strip_model_stats_recorder.h

Index: chrome/browser/ui/tabs/tab_strip_model_stats_recorder.h
diff --git a/chrome/browser/ui/tabs/tab_strip_model_stats_recorder.h b/chrome/browser/ui/tabs/tab_strip_model_stats_recorder.h
new file mode 100644
index 0000000000000000000000000000000000000000..3032fd2bd2c1eb19b5daa3cb529448e50ce5e555
--- /dev/null
+++ b/chrome/browser/ui/tabs/tab_strip_model_stats_recorder.h
@@ -0,0 +1,34 @@
+// Copyright 2015 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_UI_TABS_TAB_STRIP_MODEL_STATS_RECORDER_H_
+#define CHROME_BROWSER_UI_TABS_TAB_STRIP_MODEL_STATS_RECORDER_H_
+
+#include "base/macros.h"
+
+#include "chrome/browser/ui/browser_list_observer.h"
+#include "chrome/browser/ui/tabs/tab_strip_model_observer.h"
+
+namespace content {
+class WebContents;
+}
+
+// TabStripModelStatsRecorder records user tab interaction stats.
+// In particular, we record tab's lifetime and state transition probability to
+// study user interaction with background tabs. (crbug.com/517335)
+class TabStripModelStatsRecorder : public chrome::BrowserListObserver,
+                                   public TabStripModelObserver {
+ public:
+  TabStripModelStatsRecorder();
+  ~TabStripModelStatsRecorder() override;
+
+ private:
+  // chrome::BrowserListObserver implementation.
+  void OnBrowserAdded(Browser* browser) override;
+  void OnBrowserRemoved(Browser* browser) override;
+
+  DISALLOW_COPY_AND_ASSIGN(TabStripModelStatsRecorder);
+};
+
+#endif  // CHROME_BROWSER_UI_TABS_TAB_STRIP_MODEL_STATS_RECORDER_H_