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

ash/system/tray/system_tray_delegate.h

Index: ash/system/tray/system_tray_delegate.h
diff --git a/ash/system/tray/system_tray_delegate.h b/ash/system/tray/system_tray_delegate.h
index 9859a48885e4ff79fc05f16f507adff97c250487..9b052861d87e090f523ba1d3360bd8b28723b7a9 100644
--- a/ash/system/tray/system_tray_delegate.h
+++ b/ash/system/tray/system_tray_delegate.h
@@ -18,6 +18,8 @@
 #include "base/strings/string16.h"
 #include "ui/gfx/image/image_skia.h"
 
+class AccountId;
+
 namespace base {
 class TimeDelta;
 class TimeTicks;
@@ -314,7 +316,7 @@ class ASH_EXPORT SystemTrayDelegate {
 
   // Returns accounts delegate for given user. May return nullptr.
   virtual tray::UserAccountsDelegate* GetUserAccountsDelegate(
-      const std::string& user_id);
+      const AccountId& account_id);
 
   // Adding observers that are notified when supervised info is being changed.
   virtual void AddCustodianInfoTrayObserver(