Drive-by: Convert const to constexpr.

R=titzer@chromium.org

Bug: v8:7965
Change-Id: Ifddfba78e819a0d340fba27a6efedea654b057e8
Reviewed-on: https://chromium-review.googlesource.com/1148722Reviewed-by: Marja Hölttä <marja@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54676}