cppgc: young-gen: Return DCHECKs to AddMainThreadEvent

The DCHECKs check that the metrics from previous cycles are extracted
for both young and full GCs.

Bug: chromium:1029379
Change-Id: I8390d474abc8bd698e7f02896383b6fe013d792e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3570430Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79785}

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

@@ -298,8 +298,10 @@ void UnifiedHeapMarker::AddObject(void* object) {
 void CppHeap::MetricRecorderAdapter::AddMainThreadEvent(
     const GCCycle& cppgc_event) {
   if (cppgc_event.type == MetricRecorder::GCCycle::Type::kMinor) {
+    DCHECK(!last_young_gc_event_);
     last_young_gc_event_ = cppgc_event;
   } else {
+    DCHECK(!last_full_gc_event_);
     last_full_gc_event_ = cppgc_event;
   }
   GetIsolate()->heap()->tracer()->NotifyCppGCCompleted(