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..9a29847ea10602b8ee33d8176bb29aa022366f2d 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -5986,6 +5986,9 @@ class Map: public HeapObject {
   inline int instance_size();
   inline void set_instance_size(int value);
 
+  // Get the instance size of a map using no barrier semantics.
+  inline int nobarrier_instance_size();
+
   // Count of properties allocated in the object.
   inline int inobject_properties();
   inline void set_inobject_properties(int value);
@@ -6568,6 +6571,8 @@ class Map: public HeapObject {
 
   void TraverseTransitionTree(TraverseCallback callback, void* data);
 
+  inline void Shrink(int slack);
+
   // When you set the prototype of an object using the __proto__ accessor you
   // need a new map for the object (the prototype is stored in the map).  In
   // order not to multiply maps unnecessarily we store these as transitions in