Turn visitor_id into a byte field and free back one word in Map.

src/objects.cc

 // Copyright 2006-2009 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 2108 matching lines @@
     if (result->IsMap() && CheckHit(Map::cast(result), fast, mode)) {
 #ifdef DEBUG
       if (FLAG_enable_slow_asserts) {
         // Make sure that the new slow map has exactly the same hash as the
         // original fast map. This way we can use hash to check if a slow map
         // is already in the hash (see Contains method).
         ASSERT(Hash(fast) == Hash(Map::cast(result)));
         // The cached map should match newly created normalized map bit-by-bit.
         Object* fresh = fast->CopyNormalized(mode);
         if (!fresh->IsFailure()) {
-          // Copy the unused byte so that the assertion below works.
-          Map::cast(fresh)->address()[Map::kUnusedOffset] =
-              Map::cast(result)->address()[Map::kUnusedOffset];
           ASSERT(memcmp(Map::cast(fresh)->address(),
                         Map::cast(result)->address(),
                         Map::kSize) == 0);
         }
       }
 #endif
       return result;
     }
 
     result = fast->CopyNormalized(mode);
@@ skipping 6747 matching lines @@
   if (break_point_objects()->IsUndefined()) return 0;
   // Single beak point.
   if (!break_point_objects()->IsFixedArray()) return 1;
   // Multiple break points.
   return FixedArray::cast(break_point_objects())->length();
 }
 #endif
 
 
 } }  // namespace v8::internal