This change fixes the broken build caused by CL 23780.

Change-Id: I142cf8a1af033d036d57ac56e9e21ea925d922d4
Reviewed-on: https://go-review.googlesource.com/29490
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>