[ios] Extend WebStateListObserver amd WebStateListDelegate APIs.

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 21 matching lines @@
                              int from_index,
                              int to_index);
 
   // Invoked after the WebState at the specified index is replaced by another
   // WebState.
   virtual void WebStateReplacedAt(WebStateList* web_state_list,
                                   web::WebState* old_web_state,
                                   web::WebState* new_web_state,
                                   int index);
 
+  // Invoked before the specified WebState is detached from the WebStateList.
+  // The WebState is still valid and still in the WebStateList.
+  virtual void WillDetachWebStateAt(WebStateList* web_state_list,
+                                    web::WebState* 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 before the specified WebState is destroyed via the WebStateList.
+  // The WebState is still valid but is no longer in the WebStateList.
+  virtual void WillCloseWebStateAt(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_