Updates golang/go#13372.

Change-Id: Idfd5001f4ad7bc80a4283df1c310f97612eba85c
Reviewed-on: https://go-review.googlesource.com/23124
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>