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

chrome/browser/metrics/tab_reactivation_tracker.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_TAB_REACTIVATION_TRACKER_H_
+#define CHROME_BROWSER_METRICS_TAB_REACTIVATION_TRACKER_H_
+
+#include <memory>
+#include <unordered_map>
+
+#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 track tab deactivation/reactivation cycle. A
+// reactivation is defined as a tab that was hidden and then reshown.
+//
+// Note: A closing tab will not trigger a tab deactivation. Also worth noting
+// that tab discarding disrupt the tracking and the discarded tab will be

[Georges Khalil, 2016/09/22 18:38:38]

nit:s/disrupt/disrupts

[Patrick Monette, 2016/09/22 20:18:52]

Done.

+// treated as a new tab.
+class TabReactivationTracker : public TabStripModelObserver {
+ public:
+  class Delegate {
+   public:
+    virtual void OnTabDeactivated(content::WebContents* contents) = 0;
+    virtual void OnTabReactivated(content::WebContents* contents) = 0;
+  };
+  explicit TabReactivationTracker(Delegate* delegate);
+  ~TabReactivationTracker() override;
+
+  // 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 NotifyTabDeactivating(content::WebContents* contents);
+  void NotifyTabReactivating(content::WebContents* contents);
+
+ private:
+  class WebContentsHelper;
+
+  // Returns the helper for the |contents|, creating it if it doesn't exist.
+  WebContentsHelper* GetHelper(content::WebContents* contents);
+
+  // Called by the helper when |contents| is being destroyed.
+  void OnWebContentsDestroyed(content::WebContents* contents);
+
+  // The delegate must outlive this class.
+  Delegate* delegate_;
+
+  // A helper instance is managed per WebContents.
+  std::unordered_map<content::WebContents*, std::unique_ptr<WebContentsHelper>>
+      helper_map_;
+
+  BrowserTabStripTracker browser_tab_strip_tracker_;
+
+  DISALLOW_COPY_AND_ASSIGN(TabReactivationTracker);
+};
+
+}  // namespace metrics
+
+#endif  // CHROME_BROWSER_METRICS_TAB_REACTIVATION_TRACKER_H_