Add mojo bindings backpointer tests.

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;
@@ skipping 12 matching lines @@
   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.
+  TestFinished();  // Indicates end for echo, bit-flip, and back-pointer tests.
   PingResponse();
   EchoResponse(EchoArgsList list);
-  BitFlipResponse(EchoArgs arg);
+  BitFlipResponse(EchoArgsList arg);
+  BackPointerResponse(EchoArgsList arg);
 };
 
 [Client=CppSide]
 interface JsSide {
   Ping();
-  Echo(int32 numIterations, EchoArgsList list);
+  Echo(int32 numIterations, EchoArgs arg);
   BitFlip(EchoArgs arg);
+  BackPointer(EchoArgs arg);
 };
 
 }