Remove unused maps during marking garbage collections.

src/property.h

 // Copyright 2006-2008 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 81 matching lines @@
              int index = 0)
       : key_(key),
         value_(value),
         details_(attributes, type, index) { }
 
   friend class DescriptorWriter;
   friend class DescriptorReader;
   friend class DescriptorArray;
 };
 
-
+// A pointer from a map to the new map that is created by adding
+// a named property.  These are key to the speed and functioning of V8.
+// The two maps should always have the same prototype, since
+// MapSpace::CreateBackPointers depends on this.
 class MapTransitionDescriptor: public Descriptor {
  public:
   MapTransitionDescriptor(String* key, Map* map, PropertyAttributes attributes)
       : Descriptor(key, map, attributes, MAP_TRANSITION) { }
 };
 
 // Marks a field name in a map so that adding the field is guaranteed
 // to create a FIELD descriptor in the new map.  Used after adding
 // a constant function the first time, creating a CONSTANT_FUNCTION
 // descriptor in the new map.  This avoids creating multiple maps with
@@ skipping 267 matching lines @@
 
   // Append a descriptor to this stream.
   void Write(Descriptor* desc);
   // Read a descriptor from the reader and append it to this stream.
   void WriteFrom(DescriptorReader* reader);
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_PROPERTY_H_