This is just a minor refactoring: We include the kUnknownSectionCode in
the set of valid sections. There are only two uses of that function: The
first one already checks for kUnknownSectionCode in a separate
condition, the second one can be simplified with this refactoring.

R=ahaas@chromium.org

Bug: chromium:1335023
Change-Id: Ie1ae316db41f8111fff0af0d97a6c775f527156c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3698556
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81101}