Fix stale unhandlified value in JSObject::SetPropertyForResult.

R=danno@chromium.org
BUG=chromium:265894

Review URL: https://codereview.chromium.org/21177003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15977 ce2b1a6d-e550-0410-aec6-3dcde31c8c00