[heap] Fix condition for usage of concurrent marking deque.

src/heap/heap.cc

Index: src/heap/heap.cc
diff --git a/src/heap/heap.cc b/src/heap/heap.cc
index fc552629b6ca0695903380162876e38a611d7c99..cc85014311154bd524137e7028a5a014b9ba632e 100644
--- a/src/heap/heap.cc
+++ b/src/heap/heap.cc
@@ -5505,8 +5505,12 @@ bool Heap::SetUp() {
   mark_compact_collector_ = new MarkCompactCollector(this);
   incremental_marking_->set_marking_deque(
       mark_compact_collector_->marking_deque());
+#if V8_CONCURRENT_MARKING
   concurrent_marking_ =
       new ConcurrentMarking(this, mark_compact_collector_->marking_deque());
+#else
+  concurrent_marking_ = new ConcurrentMarking(this, nullptr);
+#endif
   if (FLAG_minor_mc)
     minor_mark_compact_collector_ = new MinorMarkCompactCollector(this);
   gc_idle_time_handler_ = new GCIdleTimeHandler();