Add filler at the new space top when forcing scavenge.

src/heap.h

Index: src/heap.h
diff --git a/src/heap.h b/src/heap.h
index 2645d27f45792888b21f90932c475a5d88ce8c1e..1ac4dfaa0808848947834983bd862aa0fc9f44d2 100644
--- a/src/heap.h
+++ b/src/heap.h
@@ -1259,10 +1259,6 @@ class Heap {
   // Notify the heap that a context has been disposed.
   int NotifyContextDisposed();
 
-  // Utility to invoke the scavenger. This is needed in test code to
-  // ensure correct callback for weak global handles.
-  void PerformScavenge();
-
   inline void increment_scan_on_scavenge_pages() {
     scan_on_scavenge_pages_++;
     if (FLAG_gc_verbose) {
@@ -2135,6 +2131,11 @@ class Heap {
   GarbageCollector SelectGarbageCollector(AllocationSpace space,
                                           const char** reason);
 
+  // Make sure there is a filler value behind the top of the new space
+  // so that the GC does not confuse some unintialized/stale memory
+  // with the allocation memento of the object at the top
+  void EnsureFillerObjectAtTop();
+
   // Performs garbage collection operation.
   // Returns whether there is a chance that another major GC could
   // collect more garbage.