[ObjC ARC] Converts ios/net to ARC.

ios/net/crn_http_url_response.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.
 
 #import "ios/net/crn_http_url_response.h"
 
 #include "base/mac/scoped_nsobject.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 @interface CRNHTTPURLResponse () {
   base::scoped_nsobject<NSString> _cr_HTTPVersion;
 }
 @end
 
 @implementation CRNHTTPURLResponse
 
 - (NSString*)cr_HTTPVersion {
   return _cr_HTTPVersion;
 }
 
 - (instancetype)initWithURL:(NSURL*)url
                  statusCode:(NSInteger)statusCode
                 HTTPVersion:(NSString*)HTTPVersion
                headerFields:(NSDictionary*)headerFields {
   self = [super initWithURL:url
                  statusCode:statusCode
                 HTTPVersion:HTTPVersion
                headerFields:headerFields];
   if (self) {
     _cr_HTTPVersion.reset([HTTPVersion copy]);
   }
   return self;
 }
 
 @end