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

BUG=chromium:718013

Review-Url: https://codereview.chromium.org/2862533003
Cr-Commit-Position: refs/heads/master@{#45067}

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();

src/heap/mark-compact.h

@@ -32,7 +32,7 @@ class PageParallelJob;
 class RecordMigratedSlotVisitor;
 class ThreadLocalTop;
 
-#ifdef V8_CONCURRENT_MARKING
+#if V8_CONCURRENT_MARKING
 using MarkingDeque = ConcurrentMarkingDeque;
 #else
 using MarkingDeque = SequentialMarkingDeque;