[heap] Add GN flag for enabling concurrent marking.

src/heap/incremental-marking.cc

 // 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.
 
 #include "src/heap/incremental-marking.h"
 
 #include "src/code-stubs.h"
 #include "src/compilation-cache.h"
 #include "src/conversions.h"
 #include "src/heap/concurrent-marking.h"
@@ skipping 123 matching lines @@
     marking_deque()->Push(obj);
     return true;
   }
   return false;
 }
 
 void IncrementalMarking::MarkBlackAndPush(HeapObject* obj) {
   // Color the object black and push it into the bailout deque.
   ObjectMarking::WhiteToGrey<kAtomicity>(obj, marking_state(obj));
   if (ObjectMarking::GreyToBlack<kAtomicity>(obj, marking_state(obj))) {
-#if V8_CONCURRENT_MARKING
+#ifdef V8_CONCURRENT_MARKING
     marking_deque()->Push(obj, MarkingThread::kMain, TargetDeque::kBailout);
 #else
     if (!marking_deque()->Push(obj)) {
       ObjectMarking::BlackToGrey<kAtomicity>(obj, marking_state(obj));
     }
 #endif
   }
 }
 
 void IncrementalMarking::TransferMark(Heap* heap, HeapObject* from,
@@ skipping 1069 matching lines @@
   idle_marking_delay_counter_++;
 }
 
 
 void IncrementalMarking::ClearIdleMarkingDelayCounter() {
   idle_marking_delay_counter_ = 0;
 }
 
 }  // namespace internal
 }  // namespace v8