[heap] Add GN flag for enabling concurrent marking.

src/globals.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_GLOBALS_H_
 #define V8_GLOBALS_H_
 
 #include <stddef.h>
 #include <stdint.h>
 
@@ skipping 100 matching lines @@
 #endif
 
 
 // Determine whether double field unboxing feature is enabled.
 #if V8_TARGET_ARCH_64_BIT
 #define V8_DOUBLE_FIELDS_UNBOXING 1
 #else
 #define V8_DOUBLE_FIELDS_UNBOXING 0
 #endif
 
-#define V8_CONCURRENT_MARKING 0
-
 // Some types of tracing require the SFI to store a unique ID.
 #if defined(V8_TRACE_MAPS) || defined(V8_TRACE_IGNITION)
 #define V8_SFI_HAS_UNIQUE_ID 1
 #endif
 
 typedef uint8_t byte;
 typedef byte* Address;
 
 // -----------------------------------------------------------------------------
 // Constants
@@ skipping 1287 matching lines @@
 }  // namespace internal
 }  // namespace v8
 
 // Used by js-builtin-reducer to identify whether ReduceArrayIterator() is
 // reducing a JSArray method, or a JSTypedArray method.
 enum class ArrayIteratorKind { kArray, kTypedArray };
 
 namespace i = v8::internal;
 
 #endif  // V8_GLOBALS_H_