Remove unused maps during marking garbage collections.

src/property.h

Index: src/property.h
===================================================================
--- src/property.h	(revision 636)
+++ src/property.h	(working copy)
@@ -99,7 +99,10 @@
   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)