[cros] Fake a stub user login when no --login-manager and --login-user are given.

chrome/browser/chromeos/login/user_manager.h

Index: chrome/browser/chromeos/login/user_manager.h
diff --git a/chrome/browser/chromeos/login/user_manager.h b/chrome/browser/chromeos/login/user_manager.h
index 60860b61f567f7a5ca2ec9b9ef7b3e2e581a95f9..419d2a2bbbe3fa51eb37b149a26163909d754af8 100644
--- a/chrome/browser/chromeos/login/user_manager.h
+++ b/chrome/browser/chromeos/login/user_manager.h
@@ -40,6 +40,9 @@ class UserManager {
   // A vector pref of the users who have logged into the device.
   static const char kLoggedInUsers[];
 
+  // Username for stub login when not running on ChromeOS.
+  static const char kStubUser[];
+
   // A dictionary that maps usernames to file paths to their wallpapers.
   // Deprecated. Will remove this const char after done migration.
   static const char kUserWallpapers[];