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/public/cpp/application/lib/application_impl_standalone.cc b/mojo/examples/apptest/example_client_impl.cc
similarity index 53%
copy from mojo/public/cpp/application/lib/application_impl_standalone.cc
copy to mojo/examples/apptest/example_client_impl.cc
index dd7f6c10e89d3d9774234de4b007c0341efed9e2..a019ffa822e2497d81bf62363c01f1a427f770b6 100644
--- a/mojo/public/cpp/application/lib/application_impl_standalone.cc
+++ b/mojo/examples/apptest/example_client_impl.cc
@@ -2,12 +2,16 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#include "mojo/public/cpp/application/application_impl.h"
+#include "mojo/examples/apptest/example_client_impl.h"
 #include "mojo/public/cpp/utility/run_loop.h"
 
 namespace mojo {
 
-void ApplicationImpl::Terminate() {
+ExampleClientImpl::ExampleClientImpl() : last_pong_value_(0) {}
+ExampleClientImpl::~ExampleClientImpl() {}
+
+void ExampleClientImpl::Pong(uint16_t pong_value) {
+  last_pong_value_ = pong_value;
   mojo::RunLoop::current()->Quit();
 }