Introduce SystemNetworkContextManager.

content/public/network/BUILD.gn

+# 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.
+
+# See //content/BUILD.gn for how this works.
+group("network") {
+  if (is_component_build) {
+    public_deps = [
+      "//content",
+    ]
+  } else {
+    public_deps = [
+      ":network_sources",
+    ]
+  }
+}
+
+target("source_set", "network_sources") {
+  visibility = [ "//content/*" ]
+
+  configs += [ "//content:content_implementation" ]
+
+  sources = [
+    "network_service.h",
+  ]
+
+  deps = [
+    "//base",
+    "//content:export",
+    "//content/common:mojo_bindings",
+    "//content/network:network_sources",
+  ]
+
+  allow_circular_includes_from = [ "//content/network:network_sources" ]
+}