GitLab

This change addresses comments left on
https://codereview.chromium.org/2277553002/ after it was checked in.

The comments were around improvements to the switch used to set the
current authenticator state.  I addressed them by removing the default
case (and leaving a comment on why) and moving the success case into the
switch.

BUG=617185

Review-Url: https://codereview.chromium.org/2308133002
Cr-Commit-Position: refs/heads/master@{#416410}