Add workaround for redefinition of __proto__ property.

This is a temporary workaround when the __proto__ property is being
redefined (e.g. by Object.freeze()) to not loose the foreign callback.
Once the __proto__ property is a real JavaScript accessor this hack is
no longer necessary. This change also makes __proto__ configurable.

R=rossberg@chromium.org
BUG=v8:2565
TEST=mjsunit/regress/regress-2565

Review URL: https://codereview.chromium.org/12398010

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13817 ce2b1a6d-e550-0410-aec6-3dcde31c8c00