CPP bindings: DCHECK when a Callback is destructed without being invoked

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

 class {{interface.name}}Stub : public mojo::MessageReceiverWithResponder {
  public:
   {{interface.name}}Stub();
   void set_sink({{interface.name}}* sink) { sink_ = sink; }
   {{interface.name}}* sink() { return sink_; }
 
   virtual bool Accept(mojo::Message* message) override;
   virtual bool AcceptWithResponder(mojo::Message* message,
                                    mojo::MessageReceiver* responder) override;
 
+  void OnConnectionError() {
+    connection_error_occurred_ = true;
+  }
+  
+  bool connection_error_occurred() const {
+    return connection_error_occurred_;
+  }
+  
+  void Close() {
+    closed_ = true;
+  }
+  
+  bool closed() {
+    return closed_;
+  }
+  
+  
  private:
   {{interface.name}}* sink_;
+  bool connection_error_occurred_;
+  bool closed_;
 };