Drive-by: Fix a bug in {ProcessParameter} which was uncovered by this CL.

R=titzer@chromium.org
CC=​​sigurds@chromium.org, jgruber@chromium.org

Bug: v8:6666
Change-Id: Ia775e8b4b9fdb9cecc226bdd870319a73950c18f
Reviewed-on: https://chromium-review.googlesource.com/c/1270589
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56477}