Fix bug in Object.isFrozen which always classifies non-extensible objects as frozen.

Fix bug in Object.isFrozen which always classifies non-extensible objects as frozen.

Since out internal representation of a property descriptor does not have configurable and writable
attributes Object.isFrozen returns true whenever an object is not extensible.
This change makes use of the right method calls on our internal representation (isWritable() and
isConfigurable()). Tests added directly to the mjsunit test.

Review URL: http://codereview.chromium.org/2904015

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@5068 ce2b1a6d-e550-0410-aec6-3dcde31c8c00