Move the notion of current Tab from TabModel to WebStateList.

ios/shared/chrome/browser/tabs/web_state_list_observer.mm

 // 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.
 
 #import "ios/shared/chrome/browser/tabs/web_state_list_observer.h"
 
 WebStateListObserver::WebStateListObserver() = default;
 
 WebStateListObserver::~WebStateListObserver() = default;
 
 void WebStateListObserver::WebStateInsertedAt(WebStateList* web_state_list,
                                               web::WebState* web_state,
                                               int index) {}
 
 void WebStateListObserver::WebStateMoved(WebStateList* web_state_list,
                                          web::WebState* web_state,
                                          int from_index,
                                          int to_index) {}
 
 void WebStateListObserver::WebStateReplacedAt(WebStateList* web_state_list,
                                               web::WebState* old_web_state,
                                               web::WebState* new_web_state,
                                               int index) {}
 
 void WebStateListObserver::WebStateDetachedAt(WebStateList* web_state_list,
                                               web::WebState* web_state,
                                               int index) {}
+
+void WebStateListObserver::WebStateActivatedAt(WebStateList* web_state_list,
+                                               web::WebState* old_web_state,
+                                               web::WebState* new_web_state,
+                                               int active_index,
+                                               bool user_action) {}