Move map retaining to finalization of incremental marking.

src/heap/heap.h

Index: src/heap/heap.h
diff --git a/src/heap/heap.h b/src/heap/heap.h
index f9435439666b9b641eff7e31aedcba416cac579d..bd1061c64737f92f50e0b327cb68edd0fdca67a2 100644
--- a/src/heap/heap.h
+++ b/src/heap/heap.h
@@ -1812,6 +1812,8 @@ class Heap {
   void AddToRingBuffer(const char* string);
   void GetFromRingBuffer(char* buffer);
 
+  void CompactRetainedMaps(ArrayList* retained_maps);
+
   // Attempt to over-approximate the weak closure by marking object groups and
   // implicit references from global handles, but don't atomically complete
   // marking. If we continue to mark incrementally, we might have marked