Make __proto__ a real JavaScript accessor property.

This turns the __proto__ callback from a foreign callback into a real
JavaScript accessor. It makes the accessor behavior of this property
explicit.

R=rossberg@chromium.org
BUG=v8:1949,v8:2606
TEST=mjsunit/regress/regress-2606

Review URL: https://codereview.chromium.org/13533004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14139 ce2b1a6d-e550-0410-aec6-3dcde31c8c00