- Sharing of descriptor arrays.
- Ensure correct enumeration indices in the dict.
- Clear the EnumLength fields of maps that lose their enumeration cache.
- Directly set the value in the descriptors pointer to avoid checks.
- CNLT with descriptors but no valid enum fields has to clear the
  EnumCache.
- Only count the descriptor array if it's owned by the map.
- Restore the descriptor array before returning allocation failure.

BUG=

Review URL: https://chromiumcodereview.appspot.com/11072014

git-svn-id: http://v8.googlecode.com/svn/trunk@12669 ce2b1a6d-e550-0410-aec6-3dcde31c8c00