Add initial NetworkService support (see issue #2622).

This change adds an --enable-network-service command-line flag to run with
NetworkService enabled.

To test: Run `cefclient --enable-network-service --disable-extensions`. The
application should start, load a website and exit without crashing.
Network-related handlers (for cookies, schemes, requests, etc) and extensions
are not expected to work yet.

There should be no functional change when running without the NetworkService
enabled.

libcef/common/net_service/util.h

@@ -0,0 +1,16 @@
+// Copyright (c) 2019 The Chromium Embedded Framework 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 CEF_LIBCEF_COMMON_NET_SERVICE_UTIL_H_
+#define CEF_LIBCEF_COMMON_NET_SERVICE_UTIL_H_
+#pragma once
+
+namespace net_service {
+
+// Returns true if the NetworkService is enabled.
+bool IsEnabled();
+
+}  // namespace net_service
+
+#endif  // CEF_LIBCEF_COMMON_NET_SERVICE_UTIL_H_