Revert "Convert `obj` ToObject in Object.keys()"

This reverts commit r24260 for breaking test262 tests.

TBR=arv@chromium.org

Review URL: https://codereview.chromium.org/608193002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24264 ce2b1a6d-e550-0410-aec6-3dcde31c8c00