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

components/user_manager/empty_user_info.cc

Index: components/user_manager/empty_user_info.cc
diff --git a/components/user_manager/empty_user_info.cc b/components/user_manager/empty_user_info.cc
index ab886f179015a84314061afea70a2e6debf7ef6f..2ecb3434b20da4769c69b3bb1fb82ce9c0846875 100644
--- a/components/user_manager/empty_user_info.cc
+++ b/components/user_manager/empty_user_info.cc
@@ -9,7 +9,7 @@
 
 namespace user_manager {
 
-EmptyUserInfo::EmptyUserInfo() {
+EmptyUserInfo::EmptyUserInfo() : user_id_(std::string() /* gaia_id */, std::string() /* email */ ) {
 }
 
 EmptyUserInfo::~EmptyUserInfo() {
@@ -30,9 +30,9 @@ std::string EmptyUserInfo::GetEmail() const {
   return std::string();
 }
 
-std::string EmptyUserInfo::GetUserID() const {
+const UserID& EmptyUserInfo::GetUserID() const {
   NOTIMPLEMENTED();
-  return std::string();
+  return user_id_;
 }
 
 const gfx::ImageSkia& EmptyUserInfo::GetImage() const {