Retain maps embedded in optimized code 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 589bebf63f8a0e57de8079d8aab7bdf9ffc768ca..82f7ee61067c8049b98c1d9ecc875a7d5c0ce93f 100644
--- a/src/heap/mark-compact.h
+++ b/src/heap/mark-compact.h
@@ -787,6 +787,10 @@ 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();
+
   // 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