cppgc: Fix benign race in unittest

Weak containers are retraced if they are found through the stack using
the conservative scanner, possibly resulting in a race with the
concurrent marker.

Bug: v8:12648
Change-Id: I0936a2953e3e2151cea4191f335a091b0e334e6b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3474678
Auto-Submit: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79184}

test/unittests/heap/cppgc/weak-container-unittest.cc

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+#include <atomic>
+
 #include "include/cppgc/allocation.h"
 #include "src/base/macros.h"
 #include "src/heap/cppgc/marker.h"
@@ -50,7 +52,10 @@ constexpr size_t SizeOf() {
 
 class TraceableGCed : public GarbageCollected<TraceableGCed> {
  public:
-  void Trace(cppgc::Visitor*) const { n_trace_calls++; }
+  void Trace(cppgc::Visitor*) const {
+    reinterpret_cast<std::atomic<size_t>*>(&n_trace_calls)
+        ->fetch_add(1, std::memory_order_relaxed);
+  }
   static size_t n_trace_calls;
 };
 size_t TraceableGCed::n_trace_calls = 0u;