Convert ios_web_shell to ARC.

ios/web/shell/shell_browser_state.mm

 // Copyright 2014 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.
 
 #include "ios/web/shell/shell_browser_state.h"
 
 #include "base/base_paths.h"
 #include "base/files/file_path.h"
 #include "base/message_loop/message_loop.h"
 #include "base/path_service.h"
 #include "base/threading/thread_restrictions.h"
 #include "ios/web/public/web_thread.h"
 #include "ios/web/shell/shell_url_request_context_getter.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 namespace web {
 
 ShellBrowserState::ShellBrowserState() : BrowserState() {
   CHECK(PathService::Get(base::DIR_APP_DATA, &path_));
 
   request_context_getter_ = new ShellURLRequestContextGetter(
       GetStatePath(),
       web::WebThread::GetTaskRunnerForThread(web::WebThread::IO),
       web::WebThread::GetTaskRunnerForThread(web::WebThread::FILE),
       web::WebThread::GetTaskRunnerForThread(web::WebThread::CACHE));
 }
 
 ShellBrowserState::~ShellBrowserState() {
 }
 
 bool ShellBrowserState::IsOffTheRecord() const {
   return false;
 }
 
 base::FilePath ShellBrowserState::GetStatePath() const {
   return path_;
 }
 
 net::URLRequestContextGetter* ShellBrowserState::GetRequestContext() {
   return request_context_getter_.get();
 }
 
 }  // namespace web