Ensure black and gray objects are kept around by scavenger

src/heap/heap.h

Index: src/heap/heap.h
diff --git a/src/heap/heap.h b/src/heap/heap.h
index 150647fbc7c0d2ad31dcc8a36ceaa8e01e54b833..9ae4406a0519f184775da05d48063a9d5c80d474 100644
--- a/src/heap/heap.h
+++ b/src/heap/heap.h
@@ -1086,6 +1086,10 @@ class Heap {
                                      Address end, bool record_slots,
                                      ObjectSlotCallback callback);
 
+  // Scavenges all objects in the from_space marked by MarkCompact while
+  // immediately promoting them.
+  void PromoteMarkedObjects();
+
   // ===========================================================================
   // Store buffer API. =========================================================
   // ===========================================================================