[heap] Add GN flag for enabling concurrent marking.

BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/config/android/config.gni")
 import("//build/config/arm.gni")
 import("//build/config/dcheck_always_on.gni")
 import("//build/config/host_byteorder.gni")
 import("//build/config/mips.gni")
 import("//build/config/sanitizers/sanitizers.gni")
@@ skipping 58 matching lines @@
 
   # Sets -dV8_TRACE_MAPS.
   v8_enable_trace_maps = ""
 
   # Sets -dV8_ENABLE_CHECKS.
   v8_enable_v8_checks = ""
 
   # Sets -dV8_TRACE_IGNITION.
   v8_enable_trace_ignition = false
 
+  # Sets -dV8_CONCURRENT_MARKING
+  v8_enable_concurrent_marking = false
+
   # With post mortem support enabled, metadata is embedded into libv8 that
   # describes various parameters of the VM for use by debuggers. See
   # tools/gen-postmortem-metadata.py for details.
   v8_postmortem_support = false
 
   # Switches off inlining in V8.
   v8_no_inline = false
 
   # Override OS page size when generating snapshot
   v8_os_page_size = "0"
@@ skipping 161 matching lines @@
   }
   if (v8_enable_handle_zapping) {
     defines += [ "ENABLE_HANDLE_ZAPPING" ]
   }
   if (v8_use_snapshot) {
     defines += [ "V8_USE_SNAPSHOT" ]
   }
   if (v8_use_external_startup_data) {
     defines += [ "V8_USE_EXTERNAL_STARTUP_DATA" ]
   }
+  if (v8_enable_concurrent_marking) {
+    defines += [ "V8_CONCURRENT_MARKING" ]
+  }
 }
 
 config("toolchain") {
   visibility = [ ":*" ]  # Only targets in this file can depend on this.
 
   defines = []
   cflags = []
   ldflags = []
 
   if (v8_current_cpu == "arm") {
@@ skipping 2999 matching lines @@
   ]
 
   configs = [
     ":external_config",
     ":internal_config_base",
   ]
 }
 
 v8_fuzzer("wasm_compile_fuzzer") {
 }