Remove the dependence on BrowserThread::FILE for

components/proxy_config/ios/proxy_service_factory.cc

Index: components/proxy_config/ios/proxy_service_factory.cc
diff --git a/components/proxy_config/ios/proxy_service_factory.cc b/components/proxy_config/ios/proxy_service_factory.cc
index 600191ae7f8babc7cf47a4df800a8cde91d971db..e0e0775a558525a28e70d227d7f96c2374fead3b 100644
--- a/components/proxy_config/ios/proxy_service_factory.cc
+++ b/components/proxy_config/ios/proxy_service_factory.cc
@@ -17,8 +17,7 @@ std::unique_ptr<net::ProxyConfigService>
 ProxyServiceFactory::CreateProxyConfigService(PrefProxyConfigTracker* tracker) {
   std::unique_ptr<net::ProxyConfigService> base_service(
       net::ProxyService::CreateSystemProxyConfigService(
-          web::WebThread::GetTaskRunnerForThread(web::WebThread::IO),
-          web::WebThread::GetTaskRunnerForThread(web::WebThread::FILE)));
+          web::WebThread::GetTaskRunnerForThread(web::WebThread::IO)));
   return tracker->CreateTrackingProxyConfigService(std::move(base_service));
 }