Mojo cpp bindings: add support for validating incoming messages.

mojo/public/cpp/bindings/lib/message_validator.cc

+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "mojo/public/cpp/bindings/lib/message_validator.h"
+
+#include <assert.h>
+
+namespace mojo {
+namespace internal {
+
+MessageValidator::MessageValidator(MessageReceiver* next) : next_(next) {
+}
+
+MessageValidator::~MessageValidator() {
+}
+
+bool MessageValidator::AcceptWithResponder(Message* message,
+                                           MessageReceiver* responder) {
+  assert(false);  // Not reached!
+  return false;
+}
+
+}  // namespace internal
+}  // namespace mojo