Mojom cpp bindings: Enable non-nullable check.

mojo/apps/js/test/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;
   uint32 ui32;
   uint16 ui16;
   uint8  ui8;
   float float_val;
   float float_inf;
   float float_nan;
   double double_val;
   double double_inf;
   double double_nan;
-  string name;
-  string[] string_array;
-  handle<message_pipe> message_handle;
-  handle<data_pipe_consumer> data_handle;
+  string? name;
+  string[]? string_array;
+  handle<message_pipe>? message_handle;
+  handle<data_pipe_consumer>? data_handle;
 };
 
 struct EchoArgsList {
-  EchoArgsList next;
-  EchoArgs item;
+  EchoArgsList? next;
+  EchoArgs? item;
 };
 
 // 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);
   BackPointerResponse(EchoArgsList arg);
 };
 
 [Client=CppSide]
 interface JsSide {
   Ping();
   Echo(int32 numIterations, EchoArgs arg);
   BitFlip(EchoArgs arg);
   BackPointer(EchoArgs arg);
 };
 
 }