[ObjC ARC] Converts ios/web:web to ARC.

ios/web/web_state/ui/wk_back_forward_list_item_holder.mm

 // Copyright 2015 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/web/web_state/ui/wk_back_forward_list_item_holder.h"
 
 #include "base/memory/ptr_util.h"
 #import "ios/web/public/navigation_item.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 namespace web {
 
 namespace {
 // Private key used for safe conversion of base::SupportsUserData to
 // web::WKBackForwardListItemHolder in
 // web::WKBackForwardListItemHolder::FromNavigationItem.
 const char kBackForwardListItemIdentifierKey[] =
     "BackForwardListItemIdentifierKey";
 }
 
@@ skipping 10 matching lines @@
       item->GetUserData(kBackForwardListItemIdentifierKey);
   if (!user_data) {
     user_data = new WKBackForwardListItemHolder();
     item->SetUserData(kBackForwardListItemIdentifierKey,
                       base::WrapUnique(user_data));
   }
   return static_cast<WKBackForwardListItemHolder*>(user_data);
 }
 
 }  // namespace web