Add a Mojo example apptest that runs in mojo_shell.

mojo/examples/apptest/example_service_impl.cc

Index: mojo/examples/apptest/example_service_impl.cc
diff --git a/mojo/examples/apptest/example_service_impl.cc b/mojo/examples/apptest/example_service_impl.cc
new file mode 100644
index 0000000000000000000000000000000000000000..5a35b23c2615ccaf304fa69367d683b4846322b8
--- /dev/null
+++ b/mojo/examples/apptest/example_service_impl.cc
@@ -0,0 +1,21 @@
+// 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.
+
+#include "mojo/examples/apptest/example_service_impl.h"
+
+namespace mojo {
+
+ExampleServiceImpl::ExampleServiceImpl() {}
+
+ExampleServiceImpl::~ExampleServiceImpl() {}
+
+void ExampleServiceImpl::Ping(uint16_t ping_value) {
+  client()->Pong(ping_value);
+}
+
+void ExampleServiceImpl::RunCallback(const Callback<void()>& callback) {
+  callback.Run();
+}
+
+}  // namespace mojo