Setting Guest last-active for the User Manager doesn't create the Guest.

chrome/browser/profiles/profiles_state.h

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CHROME_BROWSER_PROFILES_PROFILES_STATE_H_
 #define CHROME_BROWSER_PROFILES_PROFILES_STATE_H_
 
 #include <vector>
 #include "base/strings/string16.h"
 
@@ skipping 55 matching lines @@
 // If the --google-profile-info flag is turned on, starts an update for a new
 // version of the Gaia profile picture and other profile info.
 void UpdateGaiaProfileInfoIfNeeded(Profile* profile);
 
 // Returns the sign-in error controller for the given profile.  Some profiles,
 // like guest profiles, may not have a controller so this function may return
 // NULL.
 SigninErrorController* GetSigninErrorController(Profile* profile);
 
 // If the current active profile (given by prefs::kProfileLastUsed) is locked,
-// changes the active profile to the Guest profile and returns it, otherwise
-// returns NULL. This assumes that the Guest profile has been loaded.
-Profile* SetActiveProfileToGuestIfLocked();
+// changes the active profile to the Guest profile. Returns true if the active
+// profile is now guest.

[noms (inactive), 2015/01/29 16:15:13]

Hmm, this comment is a bit confusing. Why wouldn't it be guest, and is "now"
before or after the function is called? Time travel is hard :(

[Mike Lerman, 2015/01/29 16:51:40]

Done.

+bool SetActiveProfileToGuestIfLocked();
 
 }  // namespace profiles
 
 #endif  // CHROME_BROWSER_PROFILES_PROFILES_STATE_H_