Fix related corner case for Object.keys.
Remove obsolete GET_KEYS builtin.

R=ricow@chromium.org
BUG=
TEST=

Review URL: http://codereview.chromium.org/8256015

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@9760 ce2b1a6d-e550-0410-aec6-3dcde31c8c00