While a content::WebContentsUserData is attached to a tab, a chrome::TabUserData is attached to a tab (i.e. when the WebContents of a tab is replaced, the chrome::TabUserData stays the same).

Discussion: https://groups.google.com/a/google.com/d/msg/chrome-tmt/udatmO2oL9I/EGUfCW1eAgAJ