Add a Mojo example apptest that runs in mojo_shell.

mojo/examples/apptest/example_service.mojom

Index: mojo/examples/apptest/example_service.mojom
diff --git a/mojo/examples/apptest/example_service.mojom b/mojo/examples/apptest/example_service.mojom
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index 0000000000000000000000000000000000000000..a2226fa3bf8908d992d387b164bd13358009cedc
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+++ b/mojo/examples/apptest/example_service.mojom
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+// Copyright 2014 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 mojo {
+
+// A simple service used to exemplify application testing within mojo_shell.
+[Client=ExampleClient]
+interface ExampleService {
+  // Calls ExampleClient::Pong with |ping_value|.
+  Ping(uint16 ping_value);
+
+  // Runs the argument callback.
+  RunCallback() => ();
+};
+
+interface ExampleClient {
+  Pong(uint16 pong_value);
+};
+
+}