[heap] Add GN flag for enabling concurrent marking.

src/heap/heap.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/heap.h"
 
 #include "src/accessors.h"
 #include "src/api.h"
 #include "src/assembler-inl.h"
 #include "src/ast/context-slot-cache.h"
@@ skipping 5633 matching lines @@
   for (int i = 0; i < static_cast<int>(v8::Isolate::kUseCounterFeatureCount);
        i++) {
     deferred_counters_[i] = 0;
   }
 
   tracer_ = new GCTracer(this);
   scavenge_collector_ = new Scavenger(this);
   mark_compact_collector_ = new MarkCompactCollector(this);
   incremental_marking_->set_marking_deque(
       mark_compact_collector_->marking_deque());
-#if V8_CONCURRENT_MARKING
+#ifdef V8_CONCURRENT_MARKING
   concurrent_marking_ =
       new ConcurrentMarking(this, mark_compact_collector_->marking_deque());
 #else
   concurrent_marking_ = new ConcurrentMarking(this, nullptr);
 #endif
   minor_mark_compact_collector_ = new MinorMarkCompactCollector(this);
   gc_idle_time_handler_ = new GCIdleTimeHandler();
   memory_reducer_ = new MemoryReducer(this);
   if (V8_UNLIKELY(FLAG_gc_stats)) {
     live_object_stats_ = new ObjectStats(this);
@@ skipping 863 matching lines @@
     case LO_SPACE:
       return "LO_SPACE";
     default:
       UNREACHABLE();
   }
   return NULL;
 }
 
 }  // namespace internal
 }  // namespace v8