First cut at bug 992 

Fixes JS portion of DefineOwnProperty when there is 
an existing property and the new descriptor is generic. 

Makes code follow spec steps more closely. 

Fixes typo for check for unchanged enumerable in step 6. 

Adds regression test. 

Codereview url: http://codereview.chromium.org/6035014/




git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@6220 ce2b1a6d-e550-0410-aec6-3dcde31c8c00