[heap] Use atomic marking operations in incremental marking if

src/heap/incremental-marking.h

Index: src/heap/incremental-marking.h
diff --git a/src/heap/incremental-marking.h b/src/heap/incremental-marking.h
index b827f6aba17fb90411ac0c2b3911899c97475214..3b490c84eb294db9623230a3fd236e19ec395d91 100644
--- a/src/heap/incremental-marking.h
+++ b/src/heap/incremental-marking.h
@@ -161,6 +161,12 @@ class V8_EXPORT_PRIVATE IncrementalMarking {
   static const intptr_t kActivationThreshold = 0;
 #endif
 
+#ifdef V8_CONCURRENT_MARKING
+  static const MarkBit::AccessMode kAtomicity = MarkBit::AccessMode::ATOMIC;

[Hannes Payer (out of office), 2017/05/03 10:08:53]

I am not sure if this is the right mechanism here. There may be cases where we
do not run concurrent marking but use only incremental instead. The decision
will happen at runtime. Are you planning to change that later and using this as
a preparation CL only?

[ulan, 2017/05/03 12:09:30]

As discussed offline, in future we will have two IncrementalMarking instances
and use virtual dispatch on Step.

+#else
+  static const MarkBit::AccessMode kAtomicity = MarkBit::AccessMode::NON_ATOMIC;
+#endif
+
   void FinalizeSweeping();
 
   size_t Step(size_t bytes_to_process, CompletionAction action,