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

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

Index: chrome/browser/ui/cocoa/profiles/user_manager_mac_unittest.mm
diff --git a/chrome/browser/ui/cocoa/profiles/user_manager_mac_unittest.mm b/chrome/browser/ui/cocoa/profiles/user_manager_mac_unittest.mm
index 4444f1f2f412ae82b68d3b2661684f84bc10031e..760adcb940d9e5b8996852669e315c09653bce0f 100644
--- a/chrome/browser/ui/cocoa/profiles/user_manager_mac_unittest.mm
+++ b/chrome/browser/ui/cocoa/profiles/user_manager_mac_unittest.mm
@@ -4,9 +4,11 @@
 
 #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"
@@ -39,6 +41,13 @@ class UserManagerMacTest : public BrowserWithTestWindowTest {
 };
 
 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,