[heap] Fix --trace-gc-object-stats stats in debug builds

Bug: v8:11708
Change-Id: Ibf0f91b9e63646f226a2e70ec4a1733820e968ae
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3373135
Auto-Submit: Shu-yu Guo <syg@chromium.org>
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78538}

src/heap/object-stats.cc

@@ -1101,6 +1101,9 @@ class ObjectStatsVisitor {
 namespace {
 
 void IterateHeap(Heap* heap, ObjectStatsVisitor* visitor) {
+  // We don't perform a GC while collecting object stats but need this scope for
+  // the nested SafepointScope inside CombinedHeapObjectIterator.
+  AllowGarbageCollection allow_gc;
   CombinedHeapObjectIterator iterator(heap);
   for (HeapObject obj = iterator.Next(); !obj.is_null();
        obj = iterator.Next()) {