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

chrome/browser/profiles/profiles_state.cc

 // 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.
 
 #include "chrome/browser/profiles/profiles_state.h"
 
 #include "base/files/file_path.h"
 #include "base/prefs/pref_registry_simple.h"
 #include "base/prefs/pref_service.h"
 #include "base/strings/utf_string_conversions.h"
@@ skipping 153 matching lines @@
       GAIAInfoUpdateServiceFactory::GetInstance()->GetForProfile(profile);
   // The service may be null, for example during unit tests.
   if (service)
     service->Update();
 }
 
 SigninErrorController* GetSigninErrorController(Profile* profile) {
   return SigninErrorControllerFactory::GetForProfile(profile);
 }
 
-Profile* SetActiveProfileToGuestIfLocked() {
-  Profile* active_profile = ProfileManager::GetLastUsedProfile();
-  DCHECK(active_profile);
+bool SetActiveProfileToGuestIfLocked() {
+  ProfileManager* profile_manager = g_browser_process->profile_manager();
 
-  if (active_profile->IsGuestSession())
-    return active_profile;
+  const base::FilePath& active_profile_path =
+      profile_manager->GetLastUsedProfileDir(profile_manager->user_data_dir());
+  if (active_profile_path == ProfileManager::GetGuestProfilePath())

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

nit: can we store this into a variable so that we don't do the string
concatenation it does twice?

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

Done.

+    return true;
 
-  ProfileManager* profile_manager = g_browser_process->profile_manager();
   const ProfileInfoCache& cache = profile_manager->GetProfileInfoCache();
-  size_t index = cache.GetIndexOfProfileWithPath(active_profile->GetPath());
+  size_t index = cache.GetIndexOfProfileWithPath(active_profile_path);
   if (!cache.ProfileIsSigninRequiredAtIndex(index))
-    return NULL;
-
-  // Profile loads synchronously if it was not previously created.
-  Profile* guest_profile = profile_manager->GetProfile(
-      ProfileManager::GetGuestProfilePath());
-  DCHECK(guest_profile);
+    return false;
 
   PrefService* local_state = g_browser_process->local_state();
   DCHECK(local_state);
-  local_state->SetString(prefs::kProfileLastUsed,
-                         guest_profile->GetPath().BaseName().MaybeAsASCII());
-  return guest_profile;
+  local_state->SetString(
+      prefs::kProfileLastUsed,
+      ProfileManager::GetGuestProfilePath().BaseName().MaybeAsASCII());
+  return true;
 }
 
 }  // namespace profiles