heap: Fix AllocationTrackerForDebugging

Initialize thread-safe count properly.

Bug: v8:12620
Change-Id: Ifb43a860f1b8cefd410fea25ac408f5be55ab1af
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3455823
Auto-Submit: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79046}

src/heap/heap.cc

@@ -811,7 +811,7 @@ class Heap::AllocationTrackerForDebugging final
   // Count of all allocations performed through C++ bottlenecks. This needs to
   // be atomic as objects are moved in parallel in the GC which counts as
   // allocations.
-  std::atomic<size_t> allocations_count_;
+  std::atomic<size_t> allocations_count_{0};
   // Running hash over allocations performed.
   uint32_t raw_allocations_hash_ = 0;
 };