Ensure black and gray objects are kept around by scavenger

src/heap/spaces.h

Index: src/heap/spaces.h
diff --git a/src/heap/spaces.h b/src/heap/spaces.h
index e7c309e22739d8014be203ec871d0c6475e21e0c..c5683989932c93a0a977ba8e662e1f4e07f255f5 100644
--- a/src/heap/spaces.h
+++ b/src/heap/spaces.h
@@ -2861,6 +2861,8 @@ class NewSpace : public Space {
   Address ToSpaceStart() { return to_space_.space_start(); }
   Address ToSpaceEnd() { return to_space_.space_end(); }
 
+  SemiSpace* from_space() { return &from_space_; }
+
   inline bool ToSpaceContainsSlow(Address a);
   inline bool FromSpaceContainsSlow(Address a);
   inline bool ToSpaceContains(Object* o);