Move AcceptWithResponder() out of the MessageReceiver interface.

mojo/public/cpp/bindings/message.h

Index: mojo/public/cpp/bindings/message.h
diff --git a/mojo/public/cpp/bindings/message.h b/mojo/public/cpp/bindings/message.h
index e6c42058ac23d34c5183d1ccfc45e9e2fcb5e496..b8298360b0d1c3971c0df620bff824c1ec5bb9b5 100644
--- a/mojo/public/cpp/bindings/message.h
+++ b/mojo/public/cpp/bindings/message.h
@@ -84,6 +84,11 @@ class MessageReceiver {
   // was accepted and false otherwise, indicating that the message was invalid
   // or malformed.
   virtual bool Accept(Message* message) MOJO_WARN_UNUSED_RESULT = 0;
+};
+
+class MessageReceiverWithResponder : public MessageReceiver {
+ public:
+  virtual ~MessageReceiverWithResponder() {}
 
   // A variant on Accept that registers a MessageReceiver (known as the
   // responder) to handle the response message generated from the given