Fix handling of null pointers in JS bindings.

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;
 };
 
+struct EchoArgsList {
+  EchoArgsList next;
+  EchoArgs item;
+};
+
 interface CppSide {
   StartTest();  // Sent for all tests to notify that the JS side is now ready.
   TestFinished();  // Sent in echo / bit-flip tests to indicate end.
   PingResponse();
-  EchoResponse(EchoArgs arg1, EchoArgs arg2);
+  EchoResponse(EchoArgsList list);
   BitFlipResponse(EchoArgs arg);
 };
 
 [Client=CppSide]
 interface JsSide {
   Ping();
-  Echo(int32 numIterations, EchoArgs arg);
+  Echo(int32 numIterations, EchoArgsList list);
   BitFlip(EchoArgs arg);
 };
 
 }