The XXX_OneofWrappers method is a simplified way to obtain the wrapper structs
compared the previous XXX_OneofFuncs method which returned far more information
that was strictly necessary.
Change-Id: I2670506a2a8f7e8e724846b8c4083e7995371007
Reviewed-on: https://go-review.googlesource.com/c/151679
Reviewed-by: Herbie Ong <herbie@google.com>
