[ObjC ARC] Converts ios/chrome/app/startup:startup to ARC.

ios/chrome/app/startup/network_stack_setup.mm

 // Copyright 2016 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/chrome/app/startup/network_stack_setup.h"
 
 #include "base/memory/ptr_util.h"
 #include "ios/chrome/browser/application_context.h"
 #include "ios/chrome/browser/chrome_url_constants.h"
 #import "ios/net/empty_nsurlcache.h"
 #include "ios/web/net/request_tracker_factory_impl.h"
 #include "ios/web/net/web_http_protocol_handler_delegate.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 @implementation NetworkStackSetup
 
 + (void)setUpChromeNetworkStack:
             (std::unique_ptr<web::RequestTrackerFactoryImpl>*)
                 requestTrackerFactory
     httpProtocolHandlerDelegate:
         (std::unique_ptr<web::WebHTTPProtocolHandlerDelegate>*)
             httpProtocolHandlerDelegate {
   // Disable the default cache.
   [NSURLCache setSharedURLCache:[EmptyNSURLCache emptyNSURLCache]];
@@ skipping 12 matching lines @@
   BOOL success = [NSURLProtocol registerClass:[CRNHTTPProtocolHandler class]];
   DCHECK(success);
   *requestTrackerFactory =
       base::MakeUnique<web::RequestTrackerFactoryImpl>(kChromeUIScheme);
   net::RequestTracker::SetRequestTrackerFactory(requestTrackerFactory->get());
 
   DCHECK(success);
 }
 
 @end