We should use UserID object to identify users instead of username.

chrome/browser/profiles/profile_info_interface.h

 // Copyright (c) 2011 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_PROFILE_INFO_INTERFACE_H_
 #define CHROME_BROWSER_PROFILES_PROFILE_INFO_INTERFACE_H_
 
 #include "base/files/file_path.h"
 #include "base/strings/string16.h"
 
 namespace base {
 class Time;
 }
 
 namespace gfx {
 class Image;
 }
 
+namespace user_manager {
+class UserID;
+}
+
 // This abstract interface is used to query the profiles backend for information
 // about the different profiles. Its sole concrete implementation is the
 // ProfileInfoCache. This interface exists largely to assist in testing.
 class ProfileInfoInterface {
  public:
   virtual size_t GetNumberOfProfiles() const = 0;
 
   virtual size_t GetIndexOfProfileWithPath(
       const base::FilePath& profile_path) const = 0;
 
@@ skipping 36 matching lines @@
   virtual bool ProfileIsSupervisedAtIndex(size_t index) const = 0;
   // Returns whether the profile is associated with a child account.
   virtual bool ProfileIsChildAtIndex(size_t index) const = 0;
   // Returns whether the profile is a legacy supervised user profile.
   virtual bool ProfileIsLegacySupervisedAtIndex(size_t index) const = 0;
 
   // Returns true if the profile should be omitted from the desktop profile
   // list (see ProfileListDesktop), so it won't appear in the avatar menu.
   virtual bool IsOmittedProfileAtIndex(size_t index) const = 0;
 
-  virtual std::string GetSupervisedUserIdOfProfileAtIndex(
+  virtual user_manager::UserID GetSupervisedUserIdOfProfileAtIndex(
       size_t index) const = 0;
 
   // This profile is associated with an account but has been signed-out.
   virtual bool ProfileIsSigninRequiredAtIndex(size_t index) const = 0;
 
   // Profile is known to be ephemeral and should be deleted when closed.
   virtual bool ProfileIsEphemeralAtIndex(size_t index) const = 0;
 
   // Returns true if the profile is using the name it was assigned by default
   // at creation (either the old-style "Lemonade" name, or the new "Profile %d"
   // style name).
   virtual bool ProfileIsUsingDefaultNameAtIndex(size_t index) const = 0;
 
   // Returns true if the user has never manually selected a profile avatar.
   virtual bool ProfileIsUsingDefaultAvatarAtIndex(size_t index) const = 0;
 
   // Returns true if the given profile is connected to an account.
   virtual bool ProfileIsAuthenticatedAtIndex(size_t index) const = 0;
 
  protected:
   virtual ~ProfileInfoInterface() {}
 };
 
 #endif  // CHROME_BROWSER_PROFILES_PROFILE_INFO_INTERFACE_H_