heap: Fix concurrently acquiring stress marking limit

Bug: v8:12717
Change-Id: I0ffe277dda3eed2073f166f2b93a2f2c611c6d01
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3532232
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79519}

src/heap/heap.cc

@@ -5879,6 +5879,9 @@ void Heap::PrintMaxNewSpaceSizeReached() {
 }
 
 int Heap::NextStressMarkingLimit() {
+  // Reuse Heap-global mutex as this getter is called from different threads on
+  // allocation slow paths.
+  base::MutexGuard guard(relocation_mutex());
   return isolate()->fuzzer_rng()->NextInt(FLAG_stress_marking + 1);
 }