[heap] Add GN flag for enabling concurrent marking.

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index 3d03236b187fea8d6a866d610ce8281aeb507400..b203abb5aae24715f64cf0cc2ed9962a26dbb22d 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -76,6 +76,9 @@ declare_args() {
   # 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.
@@ -257,6 +260,9 @@ config("features") {
   if (v8_use_external_startup_data) {
     defines += [ "V8_USE_EXTERNAL_STARTUP_DATA" ]
   }
+  if (v8_enable_concurrent_marking) {
+    defines += [ "V8_CONCURRENT_MARKING" ]
+  }
 }
 
 config("toolchain") {