Add metrics to keep track of the tab activate/deactivate cycle

chrome/browser/metrics/tab_usage_recorder.h

Index: chrome/browser/metrics/tab_usage_recorder.h
diff --git a/chrome/browser/metrics/tab_usage_recorder.h b/chrome/browser/metrics/tab_usage_recorder.h
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+++ b/chrome/browser/metrics/tab_usage_recorder.h
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+// 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_TAB_USAGE_RECORDER_H_
+#define CHROME_BROWSER_METRICS_TAB_USAGE_RECORDER_H_
+
+#include "base/macros.h"
+#include "chrome/browser/ui/browser_tab_strip_tracker.h"
+#include "chrome/browser/ui/tabs/tab_strip_model_observer.h"
+
+namespace metrics {
+
+// This class is used to record metrics about tab reactivation. Specifically,
+// it records the number of times a tab is deactivated and reactivated. The
+// ratio of both can be calculated for tabs with different properties (e.g. If
+// the tab is pinned to the tab strip) to see if they are correlated with higher
+// probability of tab reactivation.
+class TabUsageRecorder : public TabStripModelObserver {
+ public:
+  // Starts recording tab usage for all browsers.
+  static void Initialize();
+
+  // TabStripModelObserver:
+  void TabInsertedAt(TabStripModel* tab_strip_model,
+                     content::WebContents* contents,
+                     int index,
+                     bool foreground) override;
+  void TabClosingAt(TabStripModel* tab_strip_model,
+                    content::WebContents* contents,
+                    int index) override;
+  void ActiveTabChanged(content::WebContents* old_contents,
+                        content::WebContents* new_contents,
+                        int index,
+                        int reason) override;
+  void TabPinnedStateChanged(TabStripModel* tab_strip_model,
+                             content::WebContents* contents,
+                             int index) override;
+
+ private:
+  class WebContentsData;
+
+  TabUsageRecorder();
+  ~TabUsageRecorder() override;
+
+  BrowserTabStripTracker browser_tab_strip_tracker_;
+
+  DISALLOW_COPY_AND_ASSIGN(TabUsageRecorder);
+};
+
+}  // namespace metrics
+
+#endif  // CHROME_BROWSER_METRICS_TAB_USAGE_RECORDER_H_