Updated the login flow to attempt to flush credentials

Users may want to change the logged-in user. However, most browsers cache
SSL credentials, so users will not get a chance to change their
identities. Therefore, we now attempt to flush credentials each time. The
flush should work with Chrome, at the very least.

Change-Id: I8ec36d3f79c75adeb78e754a1144b0e5419bd5c9