Reland of [heap] Fix CompactionSpace test and move to unittests

src/heap/heap.h

Index: src/heap/heap.h
diff --git a/src/heap/heap.h b/src/heap/heap.h
index 34177a4f4ba84824e777cdcbe52497a27d75bf2c..f3597db2f5f3dae4ff0fd459d6ffe0b91f1a300c 100644
--- a/src/heap/heap.h
+++ b/src/heap/heap.h
@@ -746,8 +746,8 @@ class Heap {
   // when introducing gaps within pages. If slots could have been recorded in
   // the freed area, then pass ClearRecordedSlots::kYes as the mode. Otherwise,
   // pass ClearRecordedSlots::kNo.
-  HeapObject* CreateFillerObjectAt(Address addr, int size,
-                                   ClearRecordedSlots mode);
+  V8_EXPORT_PRIVATE HeapObject* CreateFillerObjectAt(Address addr, int size,
+                                                     ClearRecordedSlots mode);
 
   bool CanMoveObjectStart(HeapObject* object);