[heap] Add GN flag for enabling concurrent marking.

src/flag-definitions.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.
 
 // This file defines all of the flags.  It is separated into different section,
 // for Debug, Release, Logging and Profiling, etc.  To add a new flag, find the
 // correct section, and use one of the DEFINE_ macros, without a trailing ';'.
 //
 // This include does not have a guard, because it is a template-style include,
 // which can be included multiple times in different modes.  It expects to have
@@ skipping 651 matching lines @@
 DEFINE_BOOL(trace_mutator_utilization, false,
             "print mutator utilization, allocation speed, gc speed")
 DEFINE_BOOL(flush_code, true, "flush code that we expect not to use again")
 DEFINE_BOOL(trace_code_flushing, false, "trace code flushing progress")
 DEFINE_BOOL(age_code, true,
             "track un-executed functions to age code and flush only "
             "old code (required for code flushing)")
 DEFINE_BOOL(incremental_marking, true, "use incremental marking")
 DEFINE_BOOL(incremental_marking_wrappers, true,
             "use incremental marking for marking wrappers")
-DEFINE_BOOL(concurrent_marking, V8_CONCURRENT_MARKING, "use concurrent marking")
+#ifdef V8_CONCURRENT_MARKING
+#define V8_CONCURRENT_MARKING_BOOL true
+#else
+#define V8_CONCURRENT_MARKING_BOOL false
+#endif
+DEFINE_BOOL(concurrent_marking, V8_CONCURRENT_MARKING_BOOL,
+            "use concurrent marking")
 DEFINE_BOOL(trace_concurrent_marking, false, "trace concurrent marking")
 DEFINE_BOOL(minor_mc_parallel_marking, true,
             "use parallel marking for the young generation")
 DEFINE_BOOL(trace_minor_mc_parallel_marking, false,
             "trace parallel marking for the young generation")
 DEFINE_INT(min_progress_during_incremental_marking_finalization, 32,
            "keep finalizing incremental marking as long as we discover at "
            "least this many unmarked objects")
 DEFINE_INT(max_incremental_marking_finalization_rounds, 3,
            "at most try this many times to finalize incremental marking")
@@ skipping 676 matching lines @@
 #undef DEFINE_ALIAS_FLOAT
 #undef DEFINE_ALIAS_ARGS
 
 #undef FLAG_MODE_DECLARE
 #undef FLAG_MODE_DEFINE
 #undef FLAG_MODE_DEFINE_DEFAULTS
 #undef FLAG_MODE_META
 #undef FLAG_MODE_DEFINE_IMPLICATIONS
 
 #undef COMMA