Revert this change again.  Somewhat mysteriously we sometimes get empty
contexts that we do not expect in the context chain.

Review URL: http://codereview.chromium.org/6372013

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@6465 ce2b1a6d-e550-0410-aec6-3dcde31c8c00