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..04ec651d483f431371b92942f887d3e9a6dcd804
--- /dev/null
+++ b/chrome/browser/ui/tabs/tab_strip_model_stats_recorder.h
@@ -0,0 +1,33 @@
+// 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 "chrome/browser/ui/browser_list_observer.h"
+#include "chrome/browser/ui/tabs/tab_strip_model_observer.h"
+
+namespace content {
+class WebContents;
+}
+
+namespace chrome {

[sky, 2015/08/11 15:27:42]

Don't use chrome namespace. If you want motivation search for threads on
chromium-dev.

[kouhei (in TOK), 2015/08/12 04:38:29]

Done.

+
+// 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 BrowserListObserver,

[sky, 2015/08/11 15:27:42]

Combine this with chrome/browser/ui/ash/metrics/chrome_user_metrics_recorder.h .

[kouhei (in TOK), 2015/08/12 04:38:28]

I'm not sure what is the best solution here.

I found ChromeUserMetricsRecorder:::OnTabSwitchedByUserGesture accessing ash
classes:
https://code.google.com/p/chromium/codesearch#chromium/src/chrome/browser/ui/...
so we can't simply move the function body.

We can add OnTabSwitchedByUserGesture callback on TabStripModelStatsRecorder so
that ash/ can inject, but it would be TabStripModelObserverObserver essentially.

+                                   public TabStripModelObserver {
+ public:
+  TabStripModelStatsRecorder();
+  ~TabStripModelStatsRecorder() override;
+
+  // chrome::BrowserListObserver implementation.

[sky, 2015/08/11 15:27:42]

Make this private, add DISALLOW_COPY_AND_ASSIGN.

[kouhei (in TOK), 2015/08/12 04:38:28]

Done.

+  void OnBrowserAdded(Browser* browser) override;
+  void OnBrowserRemoved(Browser* browser) override;
+};
+
+}  // namespace chrome
+
+#endif  // CHROME_BROWSER_UI_TABS_TAB_STRIP_MODEL_STATS_RECORDER_H_