[ios] WebStateList owns all WebState it manages.

ios/shared/chrome/browser/ui/browser_list/browser.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.h"
 
 #include "base/logging.h"
 #include "base/memory/ptr_util.h"
 #import "ios/shared/chrome/browser/tabs/web_state_list.h"
 #import "ios/shared/chrome/browser/ui/browser_list/browser_web_state_list_delegate.h"
 
 #if !defined(__has_feature) || !__has_feature(objc_arc)
 #error "This file requires ARC support."
 #endif
 
 Browser::Browser(ios::ChromeBrowserState* browser_state)
     : browser_state_(browser_state) {
   DCHECK(browser_state_);
   web_state_list_delegate_ =
       base::MakeUnique<BrowserWebStateListDelegate>(this);
-  web_state_list_ = base::MakeUnique<WebStateList>(
-      web_state_list_delegate_.get(), WebStateList::WebStateOwned);
+  web_state_list_ =
+      base::MakeUnique<WebStateList>(web_state_list_delegate_.get());
 }
 
 Browser::~Browser() = default;