Add simplistic screenlock application.

mash/shell/public/interfaces/shell.mojom

Index: mash/shell/public/interfaces/shell.mojom
diff --git a/mash/shell/public/interfaces/shell.mojom b/mash/shell/public/interfaces/shell.mojom
new file mode 100644
index 0000000000000000000000000000000000000000..7d551c4a19818457b138fc0d20a460e2beb27b5b
--- /dev/null
+++ b/mash/shell/public/interfaces/shell.mojom
@@ -0,0 +1,12 @@
+// 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.
+
+module mash.shell.mojom;
+
+interface Shell {
+  // TODO(beng): add an observer interface to observe lock changes from other
+  //             apps.
+  LockScreen();
+  UnlockScreen();
+};