This change should be relatively straightforward. It is a noop outside of the
context of xDS (as demonstrated by the fact that the tests all pass), but it
enables xDS-provided certificates (i.e. the ones that would be
provided/specified in GRPC_XDS_BOOTSTRAP). See proposal
[A29](https://github.com/grpc/proposal/blob/master/A29-xds-tls-security.md#go)
for additional detail.