Mojo: Add NotifyBadMessage API

mojo/edk/system/message_for_transit.h

Index: mojo/edk/system/message_for_transit.h
diff --git a/mojo/edk/system/message_for_transit.h b/mojo/edk/system/message_for_transit.h
index be035b04794489384453c351c0810016ec01014a..6103a771e1be5716046a33eadf55c4357998a6b5 100644
--- a/mojo/edk/system/message_for_transit.h
+++ b/mojo/edk/system/message_for_transit.h
@@ -89,6 +89,8 @@ class MOJO_SYSTEM_IMPL_EXPORT MessageForTransit {
 
   size_t num_handles() const { return header()->num_dispatchers; }
 
+  const PortsMessage& ports_message() const { return *message_; }
+
   std::unique_ptr<PortsMessage> TakePortsMessage() {
     return std::move(message_);
   }