Synchronize store buffer processing and concurrent sweeping.

src/heap.h

Index: src/heap.h
diff --git a/src/heap.h b/src/heap.h
index 758213719c3792678e8b2f89446ef6d34c53e5b9..1ac80d32563a8e72972ffd231ef8f08c8e5df142 100644
--- a/src/heap.h
+++ b/src/heap.h
@@ -1474,7 +1474,13 @@ class Heap {
   // ensure the precondition that the object is (a) a heap object and (b) in
   // the heap's from space.
   static inline void ScavengePointer(HeapObject** p);
-  static inline void ScavengeObject(HeapObject** p, HeapObject* object);
+
+  // p is the slot pointing to the object in new space
+  // object ist the object in new space
+  // old_object is not used in the ScavengeObject callback
+  static inline void ScavengeObject(HeapObject** p,
+                                    HeapObject* object,
+                                    Object* old_object = NULL);
 
   enum ScratchpadSlotMode {
     IGNORE_SCRATCHPAD_SLOT,