Mojo C++ bindings: reject messages version 2 with null payload pointer.

mojo/edk/js/tests/js_to_cpp.mojom

 module js_to_cpp;
 
 // This struct encompasses all of the basic types, so that they
 // may be sent from C++ to JS and back for validation.
 struct EchoArgs {
   int64 si64;
   int32 si32;
   int16 si16;
   int8  si8;
   uint64 ui64;
@@ skipping 10 matching lines @@
   array<string>? string_array;
   handle<message_pipe>? message_handle;
   handle<data_pipe_consumer>? data_handle;
 };
 
 struct EchoArgsList {
   EchoArgsList? next;
   EchoArgs? item;
 };
 
+interface ForTesting {};
+
 // Note: For messages which control test flow, pick numbers that are unlikely
 // to be hit as a result of our deliberate corruption of response messages.
 interface CppSide {
   // Sent for all tests to notify that the JS side is now ready.
   StartTest@88888888();
 
   // Indicates end for echo, bit-flip, and back-pointer tests.
   TestFinished@99999999();
 
   // Responses from specific tests.
   PingResponse();
   EchoResponse(EchoArgsList list);
-  BitFlipResponse(EchoArgsList arg);
+
+  // Having an associated interface pointer in the message makes sure the
+  // message header version 2 is tested.
+  BitFlipResponse(EchoArgsList arg, associated ForTesting? not_used);
+
   BackPointerResponse(EchoArgsList arg);
 };
 
 interface JsSide {
   SetCppSide(CppSide cpp);
 
   Ping();
   Echo(int32 numIterations, EchoArgs arg);
   BitFlip(EchoArgs arg);
   BackPointer(EchoArgs arg);
 };