[ios] Add a delegate to WebStateList class.

ios/clean/chrome/browser/model/browser_list.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_CLEAN_CHROME_BROWSER_MODEL_BROWSER_LIST_H_
 #define IOS_CLEAN_CHROME_BROWSER_MODEL_BROWSER_LIST_H_
 
+#include <memory>
 #include <vector>
 
 #include "base/macros.h"
 #include "base/supports_user_data.h"
 #include "ios/clean/chrome/browser/model/browser.h"
 
+class WebStateListDelegate;
+
 namespace ios {
 class ChromeBrowserState;
 }
 
 // BrowserList attaches Browsers instance to a ChromeBrowserState.
 class BrowserList : public base::SupportsUserData::Data {
  public:
   explicit BrowserList(ios::ChromeBrowserState* browser_state);
   ~BrowserList() override;
 
   static BrowserList* FromBrowserState(ios::ChromeBrowserState* browser_state);
 
   // Returns the number of open Browsers.
   int GetBrowserCount() const;
 
   // Returns whether the specified index is valid.
   int ContainsIndex(int index) const;
 
   // Returns the Browser at the specified index.
   Browser* GetBrowserAtIndex(int index) const;
 
   // Creates and returns a new Browser instance.
   Browser* CreateNewBrowser();
 
   // Closes the Browser at the specified index.
   void CloseBrowserAtIndex(int index);
 
  private:
+  ios::ChromeBrowserState* browser_state_;
+  std::unique_ptr<WebStateListDelegate> delegate_;
   std::vector<std::unique_ptr<Browser>> browsers_;
-  ios::ChromeBrowserState* browser_state_;
 
   DISALLOW_COPY_AND_ASSIGN(BrowserList);
 };
 
 #endif  // IOS_CLEAN_CHROME_BROWSER_MODEL_BROWSER_LIST_H_