Add a Mojo example apptest that runs in mojo_shell.

mojo/examples/test/example_service_impl.cc

+// 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/test/example_service_impl.h"
+
+namespace mojo {
+
+ExampleServiceImpl::ExampleServiceImpl(ApplicationConnection* connection) {}
+ExampleServiceImpl::~ExampleServiceImpl() {}
+
+void ExampleServiceImpl::Ping(uint16_t ping_value) {
+  client()->Pong(ping_value);
+}
+
+void ExampleServiceImpl::RunCallback(const Callback<void()>& callback) {
+  callback.Run();
+}
+
+}  // namespace mojo