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 67e9aae114326e8a57f55751115c063882b30712..3e607bd0a73f4df8958dae01c74630d08a004b63 100644
--- a/src/heap/spaces.h
+++ b/src/heap/spaces.h
@@ -2806,6 +2806,8 @@ 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);