Profile::IsGuestSession is set on init, not based on checking Path.
The Profile::IsGuestSession will be somewhat faster for not having
to test the Path against the user_data_dir + guest_path each time. The
method can also now be called from any thread.
This is a prelude to additional work to be done against bug 445036,
where the Profile will need to be tested for Guest status during
ProfileManager destruction, which means
ProfileManager::GetGuestSessionPath cannot be called.
BUG=445036
Review URL: https://codereview.chromium.org/
849703002
Cr-Commit-Position: refs/heads/master@{#311678}