Add chrome.power extension API.

chrome/browser/extensions/api/power/power_api_constants.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 "chrome/browser/extensions/api/power/power_api_constants.h"
+
+namespace extensions {
+namespace power_api_constants {
+
+const char kSystemLevelValue[] = "system";
+const char kDisplayLevelValue[] = "display";
+
+}  // namespace power_api_constants
+}  // namespace extensions