cppgc: young-gen: Fix copy-paste typo in marked_bytes_current()

Bug: chromium:1029379
Change-Id: I74aeaa27c27ed33c169eee74b3a8c22adedb04eb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3545320
Auto-Submit: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79595}

src/heap/cppgc/stats-collector.cc

@@ -264,7 +264,7 @@ size_t StatsCollector::marked_bytes() const {
 
 size_t StatsCollector::marked_bytes_on_current_cycle() const {
   DCHECK_NE(GarbageCollectionState::kNotRunning, gc_state_);
-  return marked_bytes_so_far_;
+  return current_.marked_bytes;
 }
 
 v8::base::TimeDelta StatsCollector::marking_time() const {