[heap] Add GN flag for enabling concurrent marking.

src/objects-inl.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.
 //
 // Review notes:
 //
 // - The use of macros in these inline functions may seem superfluous
 // but it is absolutely needed to make sure gcc generates optimal
 // code. gcc is not happy when attempting to inline too deep.
 //
@@ skipping 1285 matching lines @@
   (*reinterpret_cast<Object* const*>(FIELD_ADDR_CONST(p, offset)))
 
 #define ACQUIRE_READ_FIELD(p, offset)           \
   reinterpret_cast<Object*>(base::Acquire_Load( \
       reinterpret_cast<const base::AtomicWord*>(FIELD_ADDR_CONST(p, offset))))
 
 #define NOBARRIER_READ_FIELD(p, offset)           \
   reinterpret_cast<Object*>(base::NoBarrier_Load( \
       reinterpret_cast<const base::AtomicWord*>(FIELD_ADDR_CONST(p, offset))))
 
-#if V8_CONCURRENT_MARKING
+#ifdef V8_CONCURRENT_MARKING
 #define WRITE_FIELD(p, offset, value)                             \
   base::NoBarrier_Store(                                          \
       reinterpret_cast<base::AtomicWord*>(FIELD_ADDR(p, offset)), \
       reinterpret_cast<base::AtomicWord>(value));
 #else
 #define WRITE_FIELD(p, offset, value) \
   (*reinterpret_cast<Object**>(FIELD_ADDR(p, offset)) = value)
 #endif
 
 #define RELEASE_WRITE_FIELD(p, offset, value)                     \
@@ skipping 6944 matching lines @@
 #undef WRITE_BYTE_FIELD
 #undef NOBARRIER_READ_BYTE_FIELD
 #undef NOBARRIER_WRITE_BYTE_FIELD
 
 }  // namespace internal
 }  // namespace v8
 
 #include "src/objects/object-macros-undef.h"
 
 #endif  // V8_OBJECTS_INL_H_