{V8::Dispose} calls {FlagList::ResetAllFlags()}, which resets all flag
values. Thus the values of FLAG_dump_counters and FLAG_dump_counters_nvp
need to be read before disposing v8.

Drive-by: Two unrelated code simplifications / unifications.

R=mlippautz@chromium.org

Bug: chromium:1275117
Change-Id: Ie2bbe0c844efaf3ed50a62533c5ec08b6b423379
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3310881Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78199}