Comment 3 for bug 1605218

As David wrote, this behaviour is due to the fact that Google corporate accounts store their session in session cookies, which are normally cleared when the browser is closed.

Now, when using Chromium in the desktop, session cookies are restored, if the user has chosen that on startup the last session should be shown; in all other cases, they are cleared.
Reference:
http://stackoverflow.com/questions/10617954/chrome-doesnt-delete-session-cookies
https://bugs.chromium.org/p/chromium/issues/detail?id=128513

In our GMail (and other Google-affiliated) webapp we set the cookie mode to "restored", in order to preserve the authenticated session between restarts. That's the obvious right thing to do in a webapp.

For webbrowser-app, the behaviour is a bit more questionable, but given that webbrowser app usually starts with showing the last page the user visited, and keeps all tabs active, I would guess that the most natural behaviour would be to remember session cookies, in the same way that Chromium does when it's restoring the last visited tabs.