https://bugs.webkit.org/show_bug.cgi?id=28724

Reviewed by Gavin Barraclough.

JavaScriptCore:
Implement the core runtime support for getOwnPropertyDescriptor.
This adds a virtual getOwnPropertyDescriptor method to every class
that implements getOwnPropertySlot that shadows the behaviour of
getOwnPropertySlot.  The alternative would be to make getOwnPropertySlot
(or PropertySlots in general) provide property attribute information,
but quick testing showed this to be a regression.

WebCore:
Implement the WebCore side of getOwnPropertyDescriptor.  This
requires a custom implementation of getOwnPropertyDescriptor
for every class with a custom implementation of getOwnPropertySlot.

The bindings generator has been updated to generate appropriate
versions of getOwnPropertyDescriptor for the general case where
a custom getOwnPropertyDescriptor is not needed.  ES5 is vague
about how getOwnPropertyDescriptor should work in the context of
"host" functions with polymorphic [[GetOwnProperty]], so it seems
okay that occasionally we "guess" what attributes -- eg. determining
whether a property is writable.

Test: fast/js/getOwnPropertyDescriptor.html

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@47780 268f45cc-cd09-0410-ab3c-d52691b4dbfc