[ObjC ARC] Converts ios/net to ARC.

ios/net/empty_nsurlcache.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/net/empty_nsurlcache.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 @implementation EmptyNSURLCache
 
 + (instancetype)emptyNSURLCache {
-  return [[[EmptyNSURLCache alloc] init] autorelease];
+  return [[EmptyNSURLCache alloc] init];
 }
 
 - (NSCachedURLResponse*)cachedResponseForRequest:(NSURLRequest*)request {
   return nil;
 }
 
 @end