Use symbols instead of hidden properties for i18n markers.

Also refactor symbols in the root list.

R=dslomov@chromium.org

Review URL: https://codereview.chromium.org/614083002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24345 ce2b1a6d-e550-0410-aec6-3dcde31c8c00