Make __proto__ a foreign callback on Object.prototype.

Make __proto__ a foreign callback on Object.prototype.

This moves the __proto__ property to Object.prototype and turns it into
a callback property actually present in the descriptor array as opposed
to a hack in the properties lookup. For now it still is a "magic" data
property using foreign callbacks and not an accessor property visible to
JavaScript.

The second effect of this change is that JSON.parse() no longer treats
the __proto__ property specially, it will be defined as any other data
property. Note that object literals still have their special handling.

R=rossberg@chromium.org
BUG=v8:621,v8:1949,v8:2441
TEST=mjsunit,cctest,test262

Review URL: https://codereview.chromium.org/12212011

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@13728 ce2b1a6d-e550-0410-aec6-3dcde31c8c00