Move the notion of current Tab from TabModel to WebStateList.

ios/shared/chrome/browser/tabs/web_state_list_metrics_observer.h

 // Copyright 2017 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 IOS_SHARED_CHROME_BROWSER_TABS_WEB_STATE_LIST_METRICS_OBSERVER_H_
 #define IOS_SHARED_CHROME_BROWSER_TABS_WEB_STATE_LIST_METRICS_OBSERVER_H_
 
 #include "base/macros.h"
 #import "ios/shared/chrome/browser/tabs/web_state_list_observer.h"
 
 class WebStateListMetricsObserver : public WebStateListObserver {
  public:
   WebStateListMetricsObserver();
   ~WebStateListMetricsObserver() override;
 
+  void ResetSessionMetrics();
+  void RecordSessionMetrics();
+
   // WebStateListObserver implementation.
   void WebStateInsertedAt(WebStateList* web_state_list,
                           web::WebState* web_state,
                           int index) override;
   void WebStateReplacedAt(WebStateList* web_state_list,
                           web::WebState* old_web_state,
                           web::WebState* new_web_state,
                           int index) override;
   void WebStateDetachedAt(WebStateList* web_state_list,
                           web::WebState* web_state,
                           int index) override;
+  void WebStateActivatedAt(WebStateList* web_state_list,
+                           web::WebState* old_web_state,
+                           web::WebState* new_web_state,
+                           int active_index,
+                           bool user_action) override;
 
  private:
+  // Counters for metrics.
+  int inserted_web_state_counter_;
+  int detached_web_state_counter_;
+  int activated_web_state_counter_;
+
   DISALLOW_COPY_AND_ASSIGN(WebStateListMetricsObserver);
 };
 
 #endif  // IOS_SHARED_CHROME_BROWSER_TABS_WEB_STATE_LIST_METRICS_OBSERVER_H_