Add FlushForTesting to InterfacePtr and Binding.

mojo/public/tools/bindings/generators/cpp_templates/interface_stub_declaration.tmpl

 class {{export_attribute}} {{interface.name}}Stub : public NON_EXPORTED_BASE(mojo::MessageReceiverWithResponderStatus) {
  public:
   {{interface.name}}Stub();
   ~{{interface.name}}Stub() override;
   void set_sink({{interface.name}}* sink) { sink_ = sink; }
   {{interface.name}}* sink() { return sink_; }
   mojo::internal::SerializationContext* serialization_context() {
     return &serialization_context_;
   }
 
   bool Accept(mojo::Message* message) override;
   bool AcceptWithResponder(mojo::Message* message,
       mojo::MessageReceiverWithStatus* responder) override;
 
  private:
   {{interface.name}}* sink_;
   mojo::internal::SerializationContext serialization_context_;
-  mojo::internal::ControlMessageHandler control_message_handler_;
 };