Mojo cpp bindings: add support for validating incoming messages.

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

Index: mojo/public/cpp/bindings/lib/message_header_validator.cc
diff --git a/mojo/public/cpp/bindings/lib/message_header_validator.cc b/mojo/public/cpp/bindings/lib/message_header_validator.cc
index a5151cfd854c9612fb701984debed877613d8fbb..0239dd445e7364d649b374e0dcbdd99ebd7f9bf4 100644
--- a/mojo/public/cpp/bindings/lib/message_header_validator.cc
+++ b/mojo/public/cpp/bindings/lib/message_header_validator.cc
@@ -55,8 +55,7 @@ bool IsValidMessageHeader(const internal::MessageHeader* header) {
 }  // namespace
 
 MessageHeaderValidator::MessageHeaderValidator(MessageReceiver* next)
-    : next_(next) {
-  assert(next);
+    : MessageFilter(next) {
 }
 
 bool MessageHeaderValidator::Accept(Message* message) {
@@ -72,11 +71,5 @@ bool MessageHeaderValidator::Accept(Message* message) {
   return next_->Accept(message);
 }
 
-bool MessageHeaderValidator::AcceptWithResponder(Message* message,
-                                                 MessageReceiver* responder) {
-  assert(false);  // Not reached!
-  return false;
-}
-
 }  // namespace internal
 }  // namespace mojo