.. instead of implicitly using -1 as a marker in a few spots.

Bug: chromium:1161357
Change-Id: Icfb9a2b81dbda844c8405c57454d63ae89dfe4f9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2606336
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72037}