Revert of This CL replaces user_manager::UserID with AccountId.

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 08d4a19f84512611c8c396ae56b71d5177bb719e..ab886f179015a84314061afea70a2e6debf7ef6f 100644
--- a/components/user_manager/empty_user_info.cc
+++ b/components/user_manager/empty_user_info.cc
@@ -6,7 +6,6 @@
 
 #include "base/logging.h"
 #include "base/strings/utf_string_conversions.h"
-#include "components/signin/core/account_id/account_id.h"
 
 namespace user_manager {
 
@@ -31,9 +30,9 @@
   return std::string();
 }
 
-AccountId EmptyUserInfo::GetAccountId() const {
+std::string EmptyUserInfo::GetUserID() const {
   NOTIMPLEMENTED();
-  return EmptyAccountId();
+  return std::string();
 }
 
 const gfx::ImageSkia& EmptyUserInfo::GetImage() const {