Remove custom getOwnPropertyDescriptor for JSProxy

Reviewed by Geoff Garen.

GET_OWN_PROPERTY_DESCRIPTOR_IMPL runs afoul with JSProxy due to the workaround for JSDOMWindow's broken behavior.
Because the window object incorrectly searches the prototype chain in getOwnPropertySlot we check that the base
object matches, but in the case of JSProxy we can end up comparing the window object to the window shell & falsely
assuming this is a prototype property. Add toThis conversion to correctly identify proxied own access. I've kept
the original slotBase check as a fast case, and also so that direct access on JSDOMWindow still works.

* runtime/JSProxy.cpp:
    - Remove custom getOwnPropertyDescriptor implementation.
* runtime/PropertyDescriptor.h:
    - Modify own property access check to perform toThis conversion.



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@154334 268f45cc-cd09-0410-ab3c-d52691b4dbfc