[heap] Use atomic marking operations in incremental marking if

concurrent marking is enabled.

This patch adds kAtomicity flag to IncrementalMarking that is set
depending on the concurrent marking compile time flag.

BUG=chromium:694255

Review-Url: https://codereview.chromium.org/2857743002
Cr-Commit-Position: refs/heads/master@{#45091}

src/heap/incremental-marking.h

@@ -182,6 +182,12 @@ class V8_EXPORT_PRIVATE IncrementalMarking {
   static const intptr_t kActivationThreshold = 0;
 #endif
 
+#ifdef V8_CONCURRENT_MARKING
+  static const MarkBit::AccessMode kAtomicity = MarkBit::AccessMode::ATOMIC;
+#else
+  static const MarkBit::AccessMode kAtomicity = MarkBit::AccessMode::NON_ATOMIC;
+#endif
+
   void FinalizeSweeping();
 
   size_t Step(size_t bytes_to_process, CompletionAction action,