Fix issue with setting __proto__ on a value

LOG=N
BUG=v8:3172
R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/174113003

Patch from Erik Arvidsson <arv@chromium.org>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19666 ce2b1a6d-e550-0410-aec6-3dcde31c8c00