[ios] Extend WebStateListObserver amd WebStateListDelegate APIs.

ios/chrome/browser/tabs/tab_model_web_state_list_delegate.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_CHROME_BROWSER_TABS_TAB_MODEL_WEB_STATE_LIST_DELEGATE_H_
 #define IOS_CHROME_BROWSER_TABS_TAB_MODEL_WEB_STATE_LIST_DELEGATE_H_
 
 #import "base/ios/weak_nsobject.h"
 #include "base/macros.h"
 #import "ios/shared/chrome/browser/tabs/web_state_list_delegate.h"
 
 @class TabModel;
 
 // WebStateList delegate for the old architecture.
 class TabModelWebStateListDelegate : public WebStateListDelegate {
  public:
   explicit TabModelWebStateListDelegate(TabModel* tab_model);
   ~TabModelWebStateListDelegate() override;
 
   // WebStateListDelegate implementation.
   void WillAddWebState(web::WebState* web_state) override;
+  void WebStateDetached(web::WebState* web_state) override;
 
  private:
   base::WeakNSObject<TabModel> tab_model_;
 
   DISALLOW_COPY_AND_ASSIGN(TabModelWebStateListDelegate);
 };
 
 #endif  // IOS_CHROME_BROWSER_TABS_TAB_MODEL_WEB_STATE_LIST_DELEGATE_H_