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

ios/web/public/user_agent.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/public/user_agent.h"
 
 #import <UIKit/UIKit.h>
 
 #include <stddef.h>
 #include <stdint.h>
 #include <sys/sysctl.h>
 #include <string>
 
 #include "base/mac/scoped_nsobject.h"
 #include "base/macros.h"
 #include "base/strings/string_util.h"
 #include "base/strings/stringprintf.h"
 #include "base/strings/sys_string_conversions.h"
 #include "base/sys_info.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 namespace {
 
 struct UAVersions {
   const char* safari_version_string;
   const char* webkit_version_string;
 };
 
 struct OSVersionMap {
   int32_t major_os_version;
   int32_t minor_os_version;
@@ skipping 94 matching lines @@
                       BuildOSCpuInfo().c_str(),
                       ua_versions.webkit_version_string,
                       product.c_str(),
                       kernel_version,
                       ua_versions.safari_version_string);
 
   return user_agent;
 }
 
 }  // namespace web