Add initial prototype for the GN meta-buildsystem.

tools/gn/config_values_extractors.cc

+// Copyright (c) 2013 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.
+
+#include "tools/gn/config_values_extractors.h"
+
+namespace {
+
+struct StringWriter {
+  void operator()(const std::string& s, std::ostream& out) const {
+    out << " " << s;
+  }
+};
+
+}  // namespace
+
+void RecursiveTargetConfigStringsToStream(
+    const Target* target,
+    const std::vector<std::string>& (ConfigValues::* getter)() const,
+    std::ostream& out) {
+  RecursiveTargetConfigToStream(target, getter, StringWriter(), out);
+}