Fix bugs 992 and 1083

Fix bugs 992 and 1083

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 tests.
Fixes errors in object-define-property test

Don't normalize the JSGlobalProxy. Gets webkit http/tests/security/xss-DENIED-defineProperty.html working.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@6561 ce2b1a6d-e550-0410-aec6-3dcde31c8c00