[ios] Extend WebStateListObserver amd WebStateListDelegate APIs.

ios/shared/chrome/browser/ui/browser_list/browser_web_state_list_delegate.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/ui/browser_list/browser_web_state_list_delegate.h"
 
 #include "base/logging.h"
 
 #if !defined(__has_feature) || !__has_feature(objc_arc)
 #error "This file requires ARC support."
 #endif
 
 BrowserWebStateListDelegate::BrowserWebStateListDelegate(Browser* browser)
     : browser_(browser) {
   DCHECK(browser_);
 }
 
 BrowserWebStateListDelegate::~BrowserWebStateListDelegate() = default;
 
 void BrowserWebStateListDelegate::WillAddWebState(web::WebState* web_state) {}
+
+void BrowserWebStateListDelegate::WebStateDetached(web::WebState* web_state) {}