D8 shares counters across isolates, so even if they are only updated
from the main thread, they need to be thread-safe.
This CL removes the distinction between {StatsCounter} and
{StatsCounterThreadSafe}, and just makes all {StatsCounter} use (cheap)
atomic operations for counter updates. This will make previously
thread-safe counters cheaper, because no Mutex is involved. It might
make previously not-thread-safe counters slightly more expensive, but
it's not expected to be a significant regression.

R=mlippautz@chromium.org

Bug: v8:12481, v8:12482
Change-Id: I47b8681c1cf26d142e1ccfafa0c192e3fdcb7d2a
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/+/3320427Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78278}