[heap] Add GN flag for enabling concurrent marking.

src/heap/incremental-marking.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_INCREMENTAL_MARKING_H_
 #define V8_HEAP_INCREMENTAL_MARKING_H_
 
 #include "src/cancelable-task.h"
 #include "src/execution.h"
 #include "src/heap/heap.h"
@@ skipping 164 matching lines @@
   // This is the upper bound for how many times we allow finalization of
   // incremental marking to be postponed.
   static const int kMaxIdleMarkingDelayCounter = 3;
 
 #ifndef DEBUG
   static const intptr_t kActivationThreshold = 8 * MB;
 #else
   static const intptr_t kActivationThreshold = 0;
 #endif
 
-#if V8_CONCURRENT_MARKING
+#ifdef V8_CONCURRENT_MARKING
   static const MarkBit::AccessMode kAtomicity = MarkBit::AccessMode::ATOMIC;
 #else
   static const MarkBit::AccessMode kAtomicity = MarkBit::AccessMode::NON_ATOMIC;
 #endif
 
   void FinalizeSweeping();
 
   size_t Step(size_t bytes_to_process, CompletionAction action,
               ForceCompletionAction completion, StepOrigin step_origin);
 
@@ skipping 158 matching lines @@
 
   Observer new_generation_observer_;
   Observer old_generation_observer_;
 
   DISALLOW_IMPLICIT_CONSTRUCTORS(IncrementalMarking);
 };
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_HEAP_INCREMENTAL_MARKING_H_