httpreplay: fix race
httpreplay: fix race

The RoundTripper is accessed concurrently.

Change-Id: I5a8ea5e7833c48e925c763f3a47f511c12d0f713
Reviewed-on: https://code-review.googlesource.com/30392
Reviewed-by: Jean de Klerk <605b93664484fed1e671a07bff68da39e73d0174@google.com>
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go