This PR fixes https://github.com/vercel/next.js/issues/4883.
It introduces a new global variable (`window.__REHYDRATE_IDS`) only when necessary.
In the case of the `with-react-with-styles` example, rehydration isn't happening, so I removed the creation of the variable.
In the case of the `with-react-intl` example, the information doesn't seem to be set anymore on `__NEXT_DATA__`, so I just removed the default reference to it.
