Passes toolstash -cmp.

Updates #15756

Change-Id: Ia071dbbd7f2ee0f8433d8c37af4f7b588016244e
Reviewed-on: https://go-review.googlesource.com/38231Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>