[heap] Use a separate concurrent store buffer processing flag.

src/flag-definitions.h

Index: src/flag-definitions.h
diff --git a/src/flag-definitions.h b/src/flag-definitions.h
index e6e9b7eb7ab2ea03cf7377f955bcf930949faf74..31d55320e533a4fa4a06d8a5068da483c037330d 100644
--- a/src/flag-definitions.h
+++ b/src/flag-definitions.h
@@ -646,6 +646,8 @@ DEFINE_INT(max_incremental_marking_finalization_rounds, 3,
 DEFINE_BOOL(minor_mc, false, "perform young generation mark compact GCs")
 DEFINE_NEG_IMPLICATION(minor_mc, incremental_marking)
 DEFINE_BOOL(black_allocation, true, "use black allocation")
+DEFINE_BOOL(concurrent_store_buffer, true,
+            "use concurrent store buffer processing")
 DEFINE_BOOL(concurrent_sweeping, true, "use concurrent sweeping")
 DEFINE_BOOL(parallel_compaction, true, "use parallel compaction")
 DEFINE_BOOL(parallel_pointer_update, true,
@@ -1247,7 +1249,7 @@ DEFINE_BOOL(single_threaded, false, "disable the use of background tasks")
 DEFINE_NEG_IMPLICATION(single_threaded, concurrent_recompilation)
 DEFINE_NEG_IMPLICATION(single_threaded, concurrent_sweeping)
 DEFINE_NEG_IMPLICATION(single_threaded, parallel_compaction)
-
+DEFINE_NEG_IMPLICATION(single_threaded, concurrent_store_buffer)
 
 #undef FLAG