Mirror object properties are always names

R=aandrey@chromium.org, rossberg@chromium.org, yangguo@chromium.org
BUG=

Review URL: https://codereview.chromium.org/443843004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22959 ce2b1a6d-e550-0410-aec6-3dcde31c8c00