Use URLRequestContextBuilderMojo to create the SystemURLRequestContext.

chromeos/network/dhcp_proxy_script_fetcher_factory_chromeos.h

+// Copyright 2017 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.
+
+#ifndef CHROMEOS_NETWORK_DHCP_PROXY_SCRIPT_FETCHER_FACTORY_CHROMEOS_H_
+#define CHROMEOS_NETWORK_DHCP_PROXY_SCRIPT_FETCHER_FACTORY_CHROMEOS_H_
+
+#include <memory>
+
+#include "base/macros.h"
+#include "chromeos/chromeos_export.h"
+#include "net/proxy/dhcp_proxy_script_fetcher_factory.h"
+
+namespace net {
+class DhcpProxyScriptFetcher;
+class URLRequestContext;
+}
+
+namespace chromeos {
+
+// ChromeOS specific implementation of DhcpProxyScriptFetcherFactory.
+// TODO(mmenke):  This won't work at all with an out-of-process network service.
+// Figure out a way forward there.

[Randy Smith (Not in Mondays), 2017/06/15 19:13:30]

This isn't a problem with the Win-specific proxy script fetcher?

[mmenke, 2017/06/15 19:33:24]

On Windows, the classes are entirely within net/, so they're not a problem.  The
issue is that the ChromeOS fetcher is dependent on a bunch of ChromeOS
network-related classes that live in chrome/ and are all interdependent and have
global state.

+class CHROMEOS_EXPORT DhcpProxyScriptFetcherFactoryChromeos
+    : public net::DhcpProxyScriptFetcherFactory {
+ public:
+  DhcpProxyScriptFetcherFactoryChromeos();
+  ~DhcpProxyScriptFetcherFactoryChromeos() override;
+
+  // net::DhcpProxyScriptFetcherFactory implementation.
+  std::unique_ptr<net::DhcpProxyScriptFetcher> Create(
+      net::URLRequestContext* url_request_context) override;
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(DhcpProxyScriptFetcherFactoryChromeos);
+};
+
+}  // namespace chromeos
+
+#endif  // CHROMEOS_NETWORK_DHCP_PROXY_SCRIPT_FETCHER_FACTORY_CHROMEOS_H_