cppgc: young-gen: Gate cpp_heap->FinishSweeping behind runtime flag

Do it conditionally only when young-gen is enabled.

Change-Id: I1bd8ed49302b9e2aef0a60ed7831de9ec1cbe276
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3899308
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Auto-Submit: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83241}

src/heap/heap.cc

@@ -2497,7 +2497,11 @@ void Heap::CompleteSweepingYoung(GarbageCollector collector) {
 
 #if defined(CPPGC_YOUNG_GENERATION)
   // Always complete sweeping if young generation is enabled.
-  if (cpp_heap()) CppHeap::From(cpp_heap())->FinishSweepingIfRunning();
+  if (cpp_heap()) {
+    if (auto* iheap = CppHeap::From(cpp_heap());
+        iheap->generational_gc_supported())
+      iheap->FinishSweepingIfRunning();
+  }
 #endif  // defined(CPPGC_YOUNG_GENERATION)
 }