Add a Mojo example apptest that runs in mojo_shell.

mojo/examples/test/example_service.mojom

Index: mojo/examples/test/example_service.mojom
diff --git a/mojo/examples/test/example_service.mojom b/mojo/examples/test/example_service.mojom
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+++ b/mojo/examples/test/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 {
+
+// TODO(msw): Based on mojo/services/dbus_echo/echo.mojom ... 
+// TODO(msw): Combine with mojo/service_manager/test.mojom ??? 
+// TODO(msw): Replace with mojo/public/interfaces/bindings/tests/sample_service.mojom ??? 
+// TODO(msw): Replace with mojo/services/test_service/test_service.mojom ??? 
+[Client=ExampleClient]
+interface ExampleService {
+  // Calls the supplied callback with the supplied string.
+  Echo(string to_echo) => (string echoed);
+
+  // Calls ExampleClient::Pong with the supplied value.
+  Ping(uint16 ping_value);
+};
+
+interface ExampleClient {
+  Pong(uint16 pong_value);
+};
+
+}