Convert ios_web_shell to ARC.

ios/web/shell/shell_web_main_parts.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_web_main_parts.h"
 
 #include "ios/web/shell/shell_browser_state.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 namespace web {
 
 ShellWebMainParts::ShellWebMainParts() {
 }
 
 ShellWebMainParts::~ShellWebMainParts() {
 }
 
 void ShellWebMainParts::PreMainMessageLoopRun() {
   browser_state_.reset(new ShellBrowserState);
 }
 
 }  // namespace web