Support fast-path allocation for subclass constructors with correctly initialized initial maps.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 80a0ada87d1f4df48a0b60d0f7066780a9f33647..aa43f544eb6a0004e3b4a643c10d7b1b57fa2e9a 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -5579,6 +5579,10 @@ class Map: public HeapObject {
   static const int kRetainingCounterStart = kSlackTrackingCounterEnd - 1;
   static const int kRetainingCounterEnd = 0;
 
+  // Completes inobject slack tracking for the transition tree starting at this
+  // initial map.
+  void CompleteInobjectSlackTracking();
+
   // Tells whether the object in the prototype property will be used
   // for instances created from this function.  If the prototype
   // property is set to a value that is not a JSObject, the prototype