Add zapping of Map contents in debug mode.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 22993f25689011328c0fb2ced90bad4ba57d1689..808cf4dd07c34c5cbfc88d5a8aafeff8fa850ec9 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -4855,6 +4855,13 @@ class Map: public HeapObject {
   Handle<Map> FindTransitionedMap(MapHandleList* candidates);
   Map* FindTransitionedMap(MapList* candidates);
 
+  // Zaps the contents of backing data structures in debug mode. Note that the
+  // heap verifier (i.e. VerifyMarkingVisitor) relies on zapping of objects
+  // holding weak references when incremental marking is used.
+#ifdef DEBUG
+  void ZapInstanceDescriptors();
+  void ZapPrototypeTransitions();
+#endif
 
   // Dispatched behavior.
 #ifdef OBJECT_PRINT