Move map retaining to finalization of incremental marking.

src/heap/incremental-marking.h

Index: src/heap/incremental-marking.h
diff --git a/src/heap/incremental-marking.h b/src/heap/incremental-marking.h
index b365287c0ca69ffaf6773d1328c86b66d4c08f2e..976f4c3297b784c081d88dfdf1e93fe5decba647 100644
--- a/src/heap/incremental-marking.h
+++ b/src/heap/incremental-marking.h
@@ -241,6 +241,9 @@ class IncrementalMarking {
   void MarkRoots();
   void MarkObjectGroups();
   void ProcessWeakCells();
+  // 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();
 
   void ActivateIncrementalWriteBarrier(PagedSpace* space);
   static void ActivateIncrementalWriteBarrier(NewSpace* space);