This is a cleanup to replace some magic numbers by proper constants.

R=titzer@chromium.org

Change-Id: I549afc9688f22dd70adeb57746f6ff9ba509092c
Reviewed-on: https://chromium-review.googlesource.com/866733Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50609}