[ObjC ARC] Converts ios/web/public/test/http_server:http_server to ARC.

ios/web/public/test/http_server/string_response_provider.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/public/test/http_server/string_response_provider.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 namespace web {
 
 StringResponseProvider::StringResponseProvider(const std::string& response_body)
     : response_body_(response_body) {}
 
 bool StringResponseProvider::CanHandleRequest(const Request& request) {
   return true;
 }
 
 void StringResponseProvider::GetResponseHeadersAndBody(
     const Request& request,
     scoped_refptr<net::HttpResponseHeaders>* headers,
     std::string* response_body) {
   *headers = GetDefaultResponseHeaders();
   *response_body = response_body_;
 }
 
 }  // namespace web