Support snapshots in sky_shell (again)

sky/shell/switches.cc

+// Copyright 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 "sky/shell/switches.h"
+
+namespace sky {
+namespace shell {
+namespace switches {
+
+const char kHelp[] = "help";
+const char kPackageRoot[] = "package-root";
+const char kSnapshot[] = "snapshot";
+
+}  // namespace switches
+}  // namespace shell
+}  // namespace sky