cppgc: young-gen: Unmark only on major GCs

Bug: chromium:1029379
Change-Id: Id496cae4bb59fa247b362d17f3eaf0ecc98e9ae4
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3437051
Auto-Submit: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78940}

src/heap/cppgc-js/cpp-heap.cc

@@ -446,7 +446,9 @@ void CppHeap::InitializeTracing(
                  !GetMetricRecorder()->MetricsReportPending());
 
 #if defined(CPPGC_YOUNG_GENERATION)
-  cppgc::internal::SequentialUnmarker unmarker(raw_heap());
+  if (collection_type ==
+      cppgc::internal::GarbageCollector::Config::CollectionType::kMajor)
+    cppgc::internal::SequentialUnmarker unmarker(raw_heap());
 #endif  // defined(CPPGC_YOUNG_GENERATION)
 
   current_gc_flags_ = gc_flags;