Allow race-full access of map instance size when sweeping concurrently.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index ff9f45721f1498a181015755bb9759b670f53aa1..a77e387167b9d03b145c5732781c7202ec4600c3 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -5986,6 +5986,10 @@ class Map: public HeapObject {
   inline int instance_size();
   inline void set_instance_size(int value);
 
+  // Get and set the instance size of a map using no barrier semantics.
+  inline int nobarrier_instance_size();
+  inline void nobarrier_set_instance_size(int value);
+
   // Count of properties allocated in the object.
   inline int inobject_properties();
   inline void set_inobject_properties(int value);