Move AcceptWithResponder() out of the MessageReceiver interface.

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

-class {{interface.name}}Stub : public mojo::MessageReceiver {
+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) MOJO_OVERRIDE;
   virtual bool AcceptWithResponder(mojo::Message* message,
                                    mojo::MessageReceiver* responder)
                                        MOJO_OVERRIDE;
 
  private:
   {{interface.name}}* sink_;
 };