Keep TSAN happy by locking the string table NumberOfElements read (only
on heap counters and in the startup serializer), which can be modified
by background threads that add elements.

Bug: v8:10928
Change-Id: I411af5f9642b0cafce291344d26351ff18d2301e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2418392
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69994}