Add support for user defined "pool" to GN.

tools/gn/pool.cc

+// Copyright 2016 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/pool.h"
+
+Pool::Pool(const Settings* settings, const Label& label)
+    : Item(settings, label) {}
+
+Pool::~Pool() {}
+
+Pool* Pool::AsPool() {
+  return this;
+}
+
+const Pool* Pool::AsPool() const {
+  return this;
+}