[ObjC ARC] Converts ios/chrome/browser:browser to ARC.

ios/chrome/browser/ios_chrome_io_thread.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/chrome/browser/ios_chrome_io_thread.h"
 
 #include <stddef.h>
 
 #include <utility>
 #include <vector>
@@ skipping 61 matching lines @@
 #include "net/ssl/default_channel_id_store.h"
 #include "net/url_request/data_protocol_handler.h"
 #include "net/url_request/file_protocol_handler.h"
 #include "net/url_request/static_http_user_agent_settings.h"
 #include "net/url_request/url_request_context.h"
 #include "net/url_request/url_request_context_builder.h"
 #include "net/url_request/url_request_context_getter.h"
 #include "net/url_request/url_request_job_factory_impl.h"
 #include "url/url_constants.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 // The IOSChromeIOThread object must outlive any tasks posted to the IO thread
 // before the Quit task, so base::Bind() calls are not refcounted.
 
 namespace {
 
 const char kSupportedAuthSchemes[] = "basic,digest,ntlm";
 
 // Field trial for network quality estimator. Seeds RTT and downstream
 // throughput observations with values that correspond to the connection type
 // determined by the operating system.
@@ skipping 471 matching lines @@
 
   globals->system_http_network_session.reset(
       new net::HttpNetworkSession(system_params));
   globals->system_http_transaction_factory.reset(
       new net::HttpNetworkLayer(globals->system_http_network_session.get()));
   context->set_http_transaction_factory(
       globals->system_http_transaction_factory.get());
 
   return context;
 }