Add mojo bindings backpointer tests.

mojo/apps/js/test/js_to_cpp.mojom

Index: mojo/apps/js/test/js_to_cpp.mojom
diff --git a/mojo/apps/js/test/js_to_cpp.mojom b/mojo/apps/js/test/js_to_cpp.mojom
index 0f16f7fce978d17de8167b6223e610e4bbaf3746..7dd036cc4c1655b1174e4d64e6ad0758c60b3f58 100644
--- a/mojo/apps/js/test/js_to_cpp.mojom
+++ b/mojo/apps/js/test/js_to_cpp.mojom
@@ -30,17 +30,19 @@ struct EchoArgsList {
 
 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);
 };
 
 }