For mustash we need to replace SystemTrayDelegate. Use SessionController
for supervised user data instead.

Send a UserSession update when the Profile becomes available, because
supervised user data is kept in a profile keyed service.

Don't try to update the system tray item text while the menu is open.
The supervised user custodian almost never changes, so we'll see the new
value when the menu is opened again.

For testing, allow FakeChromeUserManager to skip the fake-addition of a
profile to a user on login. It shouldn't be doing this -- in production
the profile is loaded later.

BUG=712799
TEST=added to ash_unittests and unit_tests

Review-Url: https://codereview.chromium.org/2832903002
Cr-Commit-Position: refs/heads/master@{#466752}