Move the notion of current Tab from TabModel to WebStateList.

ios/shared/chrome/browser/tabs/web_state_list_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_OBSERVER_H_
 #define IOS_SHARED_CHROME_BROWSER_TABS_WEB_STATE_LIST_OBSERVER_H_
 
 #include "base/macros.h"
 
 class WebStateList;
@@ skipping 27 matching lines @@
                                   web::WebState* old_web_state,
                                   web::WebState* new_web_state,
                                   int index);
 
   // Invoked after the WebState at the specified index has been detached. The
   // WebState is still valid but is no longer in the WebStateList.
   virtual void WebStateDetachedAt(WebStateList* web_state_list,
                                   web::WebState* web_state,
                                   int index);
 
+  // Invoked after |new_web_state| was activated at the specified index. Both
+  // WebState are either valid or null (if there was no selection or there is
+  // no selection). If the change is due to an user action, |user_action| will
+  // be true.
+  virtual void WebStateActivatedAt(WebStateList* web_state_list,
+                                   web::WebState* old_web_state,
+                                   web::WebState* new_web_state,
+                                   int active_index,
+                                   bool user_action);
+
  private:
   DISALLOW_COPY_AND_ASSIGN(WebStateListObserver);
 };
 
 #endif  // IOS_SHARED_CHROME_BROWSER_TABS_WEB_STATE_LIST_OBSERVER_H_