Add a Mojo example apptest that runs in mojo_shell.

mojo/examples/apptest/example_client_impl.cc

Index: mojo/examples/apptest/example_client_impl.cc
diff --git a/mojo/examples/apptest/example_client_impl.cc b/mojo/examples/apptest/example_client_impl.cc
new file mode 100644
index 0000000000000000000000000000000000000000..a019ffa822e2497d81bf62363c01f1a427f770b6
--- /dev/null
+++ b/mojo/examples/apptest/example_client_impl.cc
@@ -0,0 +1,18 @@
+// 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