The order by name is maintained as secondary order by using unused bits in the property details.

This is preliminary work towards sharing descriptors arrays.

The change allows us
- to get rid of the LastAdded bits in the map, binding it to the number of valid descriptors for the given map
- to avoid resorting by enumeration index to create the cache
- (maybe in the future, depending on performance) to get rid of the enumeration cache altogether.

Although generally the number_of_descriptors equals the NumberOfOwnDescriptors in the current version, this is preliminary work towards sharing    descriptors, where maps may have more descriptors than are valid for the map.

Review URL: https://chromiumcodereview.appspot.com/10879013

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12385 ce2b1a6d-e550-0410-aec6-3dcde31c8c00