[ARC] Converts part of ios/web/public to ARC.

ios/web/public/url_schemes.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.
 
 #include "ios/web/public/url_schemes.h"
 
 #include <algorithm>
 #include <vector>
 
 #include "ios/web/public/web_client.h"
 #include "url/url_util.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 namespace web {
 
 namespace {
 void AddStandardSchemeHelper(const url::SchemeWithType& scheme) {
   url::AddStandardScheme(scheme.scheme, scheme.type);
 }
 }  // namespace
 
 void RegisterWebSchemes(bool lock_schemes) {
   std::vector<url::SchemeWithType> additional_standard_schemes;
   GetWebClient()->AddAdditionalSchemes(&additional_standard_schemes);
   std::for_each(additional_standard_schemes.begin(),
                 additional_standard_schemes.end(), AddStandardSchemeHelper);
 
   // Prevent future modification of the schemes lists. This is to prevent
   // accidental creation of data races in the program. Add*Scheme aren't
   // threadsafe so must be called when GURL isn't used on any other thread. This
   // is really easy to mess up, so we say that all calls to Add*Scheme in Chrome
   // must be inside this function.
   if (lock_schemes)
     url::LockSchemeRegistries();
 }
 
 }  // namespace web