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

chrome/browser/metrics/browser_tab_strip_observer.h

Index: chrome/browser/metrics/browser_tab_strip_observer.h
diff --git a/chrome/browser/metrics/browser_tab_strip_observer.h b/chrome/browser/metrics/browser_tab_strip_observer.h
new file mode 100644
index 0000000000000000000000000000000000000000..80c0248d830ac313337bb087e8d69e9413656af3
--- /dev/null
+++ b/chrome/browser/metrics/browser_tab_strip_observer.h
@@ -0,0 +1,36 @@
+// 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_BROWSER_TAB_STRIP_OBSERVER_H_
+#define CHROME_BROWSER_METRICS_BROWSER_TAB_STRIP_OBSERVER_H_
+
+class TabStripModel;
+
+namespace content {
+class WebContents;
+}
+
+class BrowserTabStripObserver {
+ public:
+  virtual void TabInsertedAt(TabStripModel* tab_strip_model,
+                             content::WebContents* contents,
+                             int index,
+                             bool foreground) {}
+  virtual void TabClosingAt(TabStripModel* tab_strip_model,
+                            content::WebContents* contents,
+                            int index) {}
+  virtual void ActiveTabChanged(TabStripModel* tab_strip_model,
+                                content::WebContents* old_contents,
+                                content::WebContents* new_contents,
+                                int index,
+                                int reason) {}
+  virtual void TabPinnedStateChanged(TabStripModel* tab_strip_model,
+                                     content::WebContents* contents,
+                                     int index) {}
+
+ protected:
+  virtual ~BrowserTabStripObserver() {}
+};
+
+#endif  // CHROME_BROWSER_METRICS_BROWSER_TAB_STRIP_OBSERVER_H_