[heap] Add GN flag for enabling concurrent marking.

src/heap/mark-compact.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_HEAP_MARK_COMPACT_H_
 #define V8_HEAP_MARK_COMPACT_H_
 
 #include <deque>
 
 #include "src/base/bits.h"
@@ skipping 15 matching lines @@
 class MarkCompactCollector;
 class MinorMarkCompactCollector;
 class MarkingVisitor;
 class MigrationObserver;
 template <typename JobTraits>
 class PageParallelJob;
 class RecordMigratedSlotVisitor;
 class ThreadLocalTop;
 class YoungGenerationMarkingVisitor;
 
-#if V8_CONCURRENT_MARKING
+#ifdef V8_CONCURRENT_MARKING
 using MarkingDeque = ConcurrentMarkingDeque;
 #else
 using MarkingDeque = SequentialMarkingDeque;
 #endif
 
 class ObjectMarking : public AllStatic {
  public:
   V8_INLINE static MarkBit MarkBitFrom(HeapObject* obj,
                                        const MarkingState& state) {
     const Address address = obj->address();
@@ skipping 729 matching lines @@
   ~EvacuationScope() { collector_->set_evacuation(false); }
 
  private:
   MarkCompactCollector* collector_;
 };
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_HEAP_MARK_COMPACT_H_