Counter updates were already atomic, but reading the counter values was
not. This lead to data races if one isolate called `quit` while other
isolates were still running.
This makes counters fully atomic, and reflects that by making the fields
{std::atomic<int>}.

R=mlippautz@chromium.org

Bug: v8:12481, v8:12482
Change-Id: I6fc78ad6461b93c4b3e87bec052b0a67694539e3
Cq-Include-Trybots: luci.v8.try:v8_linux64_tsan_rel_ng
Cq-Include-Trybots: luci.v8.try:v8_linux64_ubsan_rel_ng
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3320428Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78279}