Teach the native viewport service to draw a Surface into itself

mojo/examples/sample_app/sample_app.cc

Index: mojo/examples/sample_app/sample_app.cc
diff --git a/mojo/examples/sample_app/sample_app.cc b/mojo/examples/sample_app/sample_app.cc
index 6c09c0b6aac123cbc46d5ea1f4ba65a8285ef78d..26376d9ce6dd217a7fe5091802f3c8ddf84c79a6 100644
--- a/mojo/examples/sample_app/sample_app.cc
+++ b/mojo/examples/sample_app/sample_app.cc
@@ -50,6 +50,7 @@ class SampleApp : public mojo::ApplicationDelegate,
     size->width = 800;
     size->height = 600;
     mojo::CommandBufferPtr command_buffer;
+    // TODO(jamesr): Output to a surface instead.
     gpu_service_->CreateOnscreenGLES2Context(
         native_viewport_id, size.Pass(), Get(&command_buffer));
     gles2_client_.reset(new GLES2ClientImpl(command_buffer.Pass()));