Remove the dependence on BrowserThread::FILE for

ios/crnet/crnet_environment.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.
 
 #include "ios/crnet/crnet_environment.h"
 
 #import <Foundation/Foundation.h>
 
 #include <utility>
 
@@ skipping 282 matching lines @@
       base::Thread::Options(base::MessageLoop::TYPE_IO, 0));
   file_user_blocking_thread_.reset(
       new base::Thread("Chrome File User Blocking Thread"));
   file_user_blocking_thread_->StartWithOptions(
       base::Thread::Options(base::MessageLoop::TYPE_IO, 0));
 
   // The network change notifier must be initialized so that registered
   // delegates will receive callbacks.
   network_change_notifier_.reset(net::NetworkChangeNotifier::Create());
   proxy_config_service_ = net::ProxyService::CreateSystemProxyConfigService(
-      network_io_thread_->task_runner(), nullptr);
+      network_io_thread_->task_runner());
 
   main_context_getter_ = new CrNetURLRequestContextGetter(
       main_context_.get(), network_io_thread_->task_runner());
   base::subtle::MemoryBarrier();
   PostToNetworkThread(FROM_HERE,
       base::Bind(&CrNetEnvironment::InitializeOnNetworkThread,
                  base::Unretained(this)));
 
   SetRequestFilterBlock(nil);
 }
@@ skipping 198 matching lines @@
 
   return user_agent_settings->GetUserAgent();
 }
 
 void CrNetEnvironment::ClearCache(ClearCacheCallback callback) {
   PostToNetworkThread(
       FROM_HERE, base::Bind(&net::ClearHttpCache, main_context_getter_,
                             network_io_thread_->task_runner(), base::Time(),
                             base::Time::Max(), base::BindBlockArc(callback)));
 }