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

ash/session/session_state_delegate.h

 // Copyright 2015 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 ASH_SESSION_SESSION_STATE_DELEGATE_H_
 #define ASH_SESSION_SESSION_STATE_DELEGATE_H_
 
 #include <string>
 
 #include "ash/ash_export.h"
 #include "ash/session/session_types.h"
 
+class AccountId;
+
 namespace aura {
 class Window;
 }
 
 namespace gfx {
 class ImageSkia;
 }
 
 namespace user_manager {
 class UserInfo;
@@ skipping 83 matching lines @@
   // Note that |index| can at maximum be |NumberOfLoggedInUsers() - 1|.
   virtual const user_manager::UserInfo* GetUserInfo(UserIndex index) const = 0;
 
   // Whether or not the window's title should show the avatar.
   virtual bool ShouldShowAvatar(aura::Window* window) const = 0;
 
   // Returns the avatar image for the specified window.
   virtual gfx::ImageSkia GetAvatarImageForWindow(
       aura::Window* window) const = 0;
 
-  // Switches to another active user with |user_id|
+  // Switches to another active user with |account_id|
   // (if that user has already signed in).
-  virtual void SwitchActiveUser(const std::string& user_id) = 0;
+  virtual void SwitchActiveUser(const AccountId& account_id) = 0;
 
   // Switches the active user to the next or previous user, with the same
   // ordering as GetLoggedInUsers.
   virtual void CycleActiveUser(CycleUser cycle_user) = 0;
 
   // Returns true if primary user policy does not forbid multiple signin.
   virtual bool IsMultiProfileAllowedByPrimaryUserPolicy() const = 0;
 
   // Adds or removes sessions state observer.
   virtual void AddSessionStateObserver(SessionStateObserver* observer) = 0;
   virtual void RemoveSessionStateObserver(SessionStateObserver* observer) = 0;
 
   bool IsInSecondaryLoginScreen() const;
 };
 
 }  // namespace ash
 
 #endif  // ASH_SESSION_SESSION_STATE_DELEGATE_H_