Support snapshots in sky_shell (again)

sky/shell/switches.h

+// 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.
+
+#ifndef SKY_SHELL_SWITCHES_H_
+#define SKY_SHELL_SWITCHES_H_
+
+namespace sky {
+namespace shell {
+namespace switches {
+
+extern const char kHelp[];
+extern const char kPackageRoot[];
+extern const char kSnapshot[];
+
+}  // namespace switches
+}  // namespace shell
+}  // namespace sky
+
+#endif  // SKY_SHELL_SWITCHES_H_