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

chrome/browser/ui/cocoa/profiles/user_manager_mac_unittest.mm

 // Copyright 2014 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/ui/cocoa/profiles/user_manager_mac.h"
 
+#include "base/prefs/pref_service.h"
 #include "base/run_loop.h"
 #include "chrome/browser/profiles/profile_window.h"
 #include "chrome/browser/ui/user_manager.h"
+#include "chrome/common/pref_names.h"
 #include "chrome/test/base/browser_with_test_window_test.h"
 #include "chrome/test/base/testing_browser_process.h"
 #include "chrome/test/base/testing_profile_manager.h"
 
 class UserManagerMacTest : public BrowserWithTestWindowTest {
  public:
   UserManagerMacTest()
       : testing_profile_manager_(TestingBrowserProcess::GetGlobal()) {
   }
 
@@ skipping 12 matching lines @@
     BrowserWithTestWindowTest::TearDown();
   }
 
  private:
   TestingProfileManager testing_profile_manager_;
 
   DISALLOW_COPY_AND_ASSIGN(UserManagerMacTest);
 };
 
 TEST_F(UserManagerMacTest, ShowUserManager) {
+  // Set the ProfileLastUsed pref so that SetActiveProfileToGuestIfLocked()
+  // uses a last active profile that's in the ProfileInfoCache, not default.
+  g_browser_process->local_state()->SetString(
+      prefs::kProfileLastUsed,
+      g_browser_process->profile_manager()->GetProfileInfoCache().
+          GetPathOfProfileAtIndex(0).BaseName().MaybeAsASCII());
+
   EXPECT_FALSE(UserManager::IsShowing());
   UserManager::Show(base::FilePath(),
                     profiles::USER_MANAGER_NO_TUTORIAL,
                     profiles::USER_MANAGER_SELECT_PROFILE_NO_ACTION);
   EXPECT_TRUE(UserManager::IsShowing());
 
   UserManager::Hide();
   EXPECT_FALSE(UserManager::IsShowing());
 }