Remove the dependence on BrowserThread::FILE for

headless/lib/browser/headless_url_request_context_getter.cc

Index: headless/lib/browser/headless_url_request_context_getter.cc
diff --git a/headless/lib/browser/headless_url_request_context_getter.cc b/headless/lib/browser/headless_url_request_context_getter.cc
index cf3d9b336d38d4f5dd7fda858c18b6e9b9fa4d68..ef04cb92300d445d644b2e1d3799df8a9e9a6f1e 100644
--- a/headless/lib/browser/headless_url_request_context_getter.cc
+++ b/headless/lib/browser/headless_url_request_context_getter.cc
@@ -51,8 +51,8 @@ HeadlessURLRequestContextGetter::HeadlessURLRequestContextGetter(
   // must synchronously run on the glib message loop. This will be passed to
   // the URLRequestContextStorage on the IO thread in GetURLRequestContext().
   if (proxy_server_.IsEmpty()) {
-    proxy_config_service_ = net::ProxyService::CreateSystemProxyConfigService(
-        io_task_runner_, file_task_runner_);
+    proxy_config_service_ =
+        net::ProxyService::CreateSystemProxyConfigService(io_task_runner_);
   }
 }