Add a Mojo example apptest that runs in mojo_shell.

mojo/examples/apptest/example_client_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/apptest/example_client_impl.h"
+#include "mojo/public/cpp/utility/run_loop.h"
+
+namespace mojo {
+
+ExampleClientImpl::ExampleClientImpl() : last_pong_value_(0) {}
+ExampleClientImpl::~ExampleClientImpl() {}
+
+void ExampleClientImpl::Pong(uint16_t pong_value) {
+  last_pong_value_ = pong_value;
+  mojo::RunLoop::current()->Quit();
+}
+
+}  // namespace mojo