By popular demand: Slightly more honest typing in ClearNonLiveMapTransitions

Review URL: https://chromiumcodereview.appspot.com/9290049

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@10526 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/mark-compact.cc

@@ -2368,9 +2368,9 @@ void MarkCompactCollector::ClearNonLivePrototypeTransitions(Map* map) {
 void MarkCompactCollector::ClearNonLiveMapTransitions(Map* map,
                                                       MarkBit map_mark) {
   // Follow the chain of back pointers to find the prototype.
-  Map* real_prototype = map;
+  Object* real_prototype = map;
   while (real_prototype->IsMap()) {
-    real_prototype = reinterpret_cast<Map*>(real_prototype->prototype());
+    real_prototype = Map::cast(real_prototype)->prototype();
     ASSERT(real_prototype->IsHeapObject());
   }