Keys are expected to be tenured. This now not only includes internalized
strings, but also symbols.

R=rossberg@chromium.org
BUG=

Review URL: https://chromiumcodereview.appspot.com/13158002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@14095 ce2b1a6d-e550-0410-aec6-3dcde31c8c00