This CL replaces std::string user_id in ash/* with AccountId.

ash/system/tray/system_tray_delegate.cc

Index: ash/system/tray/system_tray_delegate.cc
diff --git a/ash/system/tray/system_tray_delegate.cc b/ash/system/tray/system_tray_delegate.cc
index 314817eefe5aa2137d6b84a4f43ee7a0486fda4b..57735de0751bb65a02ce274ca026fdb9aadf1d0e 100644
--- a/ash/system/tray/system_tray_delegate.cc
+++ b/ash/system/tray/system_tray_delegate.cc
@@ -259,7 +259,7 @@ bool SystemTrayDelegate::IsSearchKeyMappedToCapsLock() {
 }
 
 tray::UserAccountsDelegate* SystemTrayDelegate::GetUserAccountsDelegate(
-    const std::string& user_id) {
+    const AccountId& /* account_id */) {
   return nullptr;
 }