Introduce SystemNetworkContextManager.

content/public/browser/network_service_instance.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 CONTENT_PUBLIC_BROWSER_NETWORK_SERVICE_INSTANCE_H_
+#define CONTENT_PUBLIC_BROWSER_NETWORK_SERVICE_INSTANCE_H_
+
+#include "content/common/content_export.h"
+
+namespace content {
+
+namespace mojom {
+class NetworkService;
+}
+
+// Returns a pointer to the NetworkService, creating / re-creating it as needed.
+// Must only be called on the UI thread. Must not be called if the network
+// service is disabled.
+CONTENT_EXPORT mojom::NetworkService* GetNetworkService();
+
+}  // namespace content
+
+#endif  // CONTENT_PUBLIC_BROWSER_NETWORK_SERVICE_INSTANCE_H_