The healthiest and the easiest way to end FTP session in Google Chrome might be only using Chrome Incognito mode for FTP processes, then closing it when finished those FTP processes.
You can launch incognito by pressing Ctrl + Shift + N, (or) Settings -> New incognito Window, (or) right mouse-click on a link from a regular Chrome session, and select “open link in incognito window”.
Technically ending a FTP session in Google Chrome is complicated or maybe impossible. It only lets you keep connected to FTP server during a single browser session. In most of other browsers this situation is controllable. For example; You can resume to download a file from a FTP server even you have rebooted the system in Firefox FTP session.
Waiting a while to end the FTP session is not also a solution, because cache files are not deleted when Chrome returns ERR_TIMED_OUT. Refreshing page brings credentials of the FTP session back.
Unfortunatelly, there is no suitable solution yet for Google Chrome that I know.