Set iIsExtensible to true as default in the map.

Review URL: http://codereview.chromium.org/556041

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3718 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/heap.cc

@@ -1206,7 +1206,7 @@ Object* Heap::AllocateMap(InstanceType instance_type, int instance_size) {
   map->set_code_cache(empty_fixed_array());
   map->set_unused_property_fields(0);
   map->set_bit_field(0);
-  map->set_bit_field2(0);
+  map->set_bit_field2(1 << Map::kIsExtensible);
 
   // If the map object is aligned fill the padding area with Smi 0 objects.
   if (Map::kPadStart < Map::kSize) {