Add support for getOwnPropertyDescriptor on array indices (fixes issue 599).

Add support for getOwnPropertyDescriptor on array indices (fixes issue 599).

This fix adds support for retriving a property descriptor on elements. The
new version supports both fast and slow case elements. In the fast case
we always default configurable, writable, enumerable to true (we don't have
PropertyDetails for fast elements).

A few new tests are added to get-own-property-descriptor.js, I will
add a lot more to object-define-property when I add support for indices in
Object.defineProperty.

Review URL: http://codereview.chromium.org/2278002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@4738 ce2b1a6d-e550-0410-aec6-3dcde31c8c00