Use atomic operation to read the length of a fixed array.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index cf9184ffe128f7da53051fe2939ee2e13a057f9d..04aee1ba41d1edcbd8d53ce60a7594a61b65d7ce 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -2521,7 +2521,8 @@ class FixedArray: public FixedArrayBase {
   class BodyDescriptor : public FlexibleBodyDescriptor<kHeaderSize> {
    public:
     static inline int SizeOf(Map* map, HeapObject* object) {
-      return SizeFor(reinterpret_cast<FixedArray*>(object)->length());
+      return SizeFor(
+          reinterpret_cast<FixedArray*>(object)->synchronized_length());
     }
   };