Refactor copying of maps and descriptor arrays.

src/heap.cc

Index: src/heap.cc
diff --git a/src/heap.cc b/src/heap.cc
index 8e724ebb6de99674ebd72883f1baa5ac30372899..a448292789ad72511cb6c6922dc903bc1f14eb8a 100644
--- a/src/heap.cc
+++ b/src/heap.cc
@@ -4180,10 +4180,10 @@ MaybeObject* Heap::AllocateGlobalObject(JSFunction* constructor) {
   InitializeJSObjectFromMap(global, dictionary, map);
 
   // Create a new map for the global object.
-  { MaybeObject* maybe_obj = map->CopyDropDescriptors();
-    if (!maybe_obj->ToObject(&obj)) return maybe_obj;
+  Map* new_map;
+  { MaybeObject* maybe_map = map->CopyDropDescriptors();
+    if (!maybe_map->To(&new_map)) return maybe_map;
   }
-  Map* new_map = Map::cast(obj);
 
   // Set up the global object as a normalized object.
   global->set_map(new_map);