Add initial prototype for the GN meta-buildsystem.

tools/gn/item.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/item.h"
+
+#include "base/logging.h"
+
+Item::Item(const Label& label) : label_(label) {
+}
+
+Item::~Item() {
+}
+
+Config* Item::AsConfig() { return NULL; }
+const Config* Item::AsConfig() const { return NULL; }
+Target* Item::AsTarget() { return NULL; }
+const Target* Item::AsTarget() const { return NULL; }
+Toolchain* Item::AsToolchain() { return NULL; }
+const Toolchain* Item::AsToolchain() const { return NULL; }
+
+std::string Item::GetItemTypeName() const {
+  if (AsConfig())
+    return "config";
+  if (AsTarget())
+    return "target";
+  if (AsToolchain())
+    return "toolchain";
+  NOTREACHED();
+  return "this thing that I have no idea what it is";
+}