Pass file task runner to net::URLRequestContextBuilder

This avoids creation of an extra file thread in BasicURLRequestContext
and fixes linked bug

BUG=475213

Review URL: https://codereview.chromium.org/1082803004

Cr-Commit-Position: refs/heads/master@{#327082}

remoting/base/url_request_context_getter.cc

@@ -15,7 +15,8 @@ namespace remoting {
 URLRequestContextGetter::URLRequestContextGetter(
     scoped_refptr<base::SingleThreadTaskRunner> network_task_runner,
     scoped_refptr<base::SingleThreadTaskRunner> file_task_runner)
-    : network_task_runner_(network_task_runner) {
+    : network_task_runner_(network_task_runner),
+      file_task_runner_(file_task_runner) {
   proxy_config_service_.reset(net::ProxyService::CreateSystemProxyConfigService(
       network_task_runner_, file_task_runner));
 }
@@ -23,6 +24,7 @@ URLRequestContextGetter::URLRequestContextGetter(
 net::URLRequestContext* URLRequestContextGetter::GetURLRequestContext() {
   if (!url_request_context_.get()) {
     net::URLRequestContextBuilder builder;
+    builder.SetFileTaskRunner(file_task_runner_);
     builder.set_net_log(new VlogNetLog());
     builder.DisableHttpCache();
     builder.set_proxy_config_service(proxy_config_service_.release());

remoting/base/url_request_context_getter.h

@@ -35,6 +35,7 @@ class URLRequestContextGetter : public net::URLRequestContextGetter {
 
  private:
   scoped_refptr<base::SingleThreadTaskRunner> network_task_runner_;
+  scoped_refptr<base::SingleThreadTaskRunner> file_task_runner_;
   scoped_ptr<net::ProxyConfigService> proxy_config_service_;
   scoped_ptr<net::URLRequestContext> url_request_context_;