Follow-up to review feedback from
mdempsky on CL 40507.

Reduces mutex contention by about 1%.

Change-Id: I540ea6772925f4a59e58f55a3458eff15880c328
Reviewed-on: https://go-review.googlesource.com/40575
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>