Improve readability by avoiding a helper method. Just read from the
stack directly.

R=titzer@chromium.org

Change-Id: I38c944fac45c721f328a2b7bec3a3f4602f05c05
Reviewed-on: https://chromium-review.googlesource.com/c/1360572
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58023}