[ios] Extend WebStateListObserver amd WebStateListDelegate APIs.

ios/shared/chrome/browser/tabs/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_SHARED_CHROME_BROWSER_TABS_WEB_STATE_LIST_DELEGATE_H_
 #define IOS_SHARED_CHROME_BROWSER_TABS_WEB_STATE_LIST_DELEGATE_H_
 
 #include "base/macros.h"
 
 namespace web {
 class WebState;
 }
 
 // A delegate interface that the WebStateList uses to perform work that it
 // cannot do itself such as attaching tab helpers, ...
 class WebStateListDelegate {
  public:
   WebStateListDelegate() = default;
   virtual ~WebStateListDelegate() = default;
 
   // Notifies the delegate that the specified WebState will be added to the
   // WebStateList (via insertion/appending/replacing existing) and allows it
   // to do any preparation that it deems necessary.
   virtual void WillAddWebState(web::WebState* web_state) = 0;
 
+  // Notifies the delegate that the specified WebState has been detached from
+  // the WebStateList (via detaching/closing/replacing existing) and allows it
+  // to do any clean up that it deems necessary.
+  virtual void WebStateDetached(web::WebState* web_state) = 0;
+
  private:
   DISALLOW_COPY_AND_ASSIGN(WebStateListDelegate);
 };
 
 #endif  // IOS_SHARED_CHROME_BROWSER_TABS_WEB_STATE_LIST_DELEGATE_H_