Retain maps for several garbage collections

src/heap/mark-compact.h

Index: src/heap/mark-compact.h
diff --git a/src/heap/mark-compact.h b/src/heap/mark-compact.h
index e5ab308bef178834825423b2f9181f29a7a99b77..c11851847b661f791a82f2f31bc777cbafad78de 100644
--- a/src/heap/mark-compact.h
+++ b/src/heap/mark-compact.h
@@ -780,6 +780,10 @@ class MarkCompactCollector {
   // otherwise a map can die and deoptimize the code.
   void ProcessTopOptimizedFrame(ObjectVisitor* visitor);
 
+  // Retain dying maps for <FLAG_retain_maps_for_n_gc> garbage collections to
+  // increase chances of reusing of map transition tree in future.
+  void RetainMaps(ObjectVisitor* visitor);
+
   // Mark objects reachable (transitively) from objects in the marking
   // stack.  This function empties the marking stack, but may leave
   // overflowed objects in the heap, in which case the marking stack's