Change some vars to consts, remove some unneeded string conversions.
Change-Id: Ib12eed11ef080c4b593c8369bb915117e7100045
Reviewed-on: https://go-review.googlesource.com/c/go/+/621838
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Dmitri Shuralyov <dmitshur@google.com> Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com> Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>