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

ash/shell/shell_delegate_impl.cc

 // Copyright (c) 2012 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.
 
 #include "ash/shell/shell_delegate_impl.h"
 
 #include "ash/accessibility_delegate.h"
 #include "ash/default_accessibility_delegate.h"
 #include "ash/default_user_wallpaper_delegate.h"
 #include "ash/gpu_support_stub.h"
@@ skipping 99 matching lines @@
   }
   const user_manager::UserInfo* GetUserInfo(UserIndex index) const override {
     return user_info_.get();
   }
   bool ShouldShowAvatar(aura::Window* window) const override {
     return !user_info_->GetImage().isNull();
   }
   gfx::ImageSkia GetAvatarImageForWindow(aura::Window* window) const override {
     return gfx::ImageSkia();
   }
-  void SwitchActiveUser(const std::string& user_id) override {}
+  void SwitchActiveUser(const AccountId& account_id) override {}
   void CycleActiveUser(CycleUser cycle_user) override {}
   bool IsMultiProfileAllowedByPrimaryUserPolicy() const override {
     return true;
   }
   void AddSessionStateObserver(ash::SessionStateObserver* observer) override {}
   void RemoveSessionStateObserver(
       ash::SessionStateObserver* observer) override {}
 
  private:
   bool screen_locked_;
@@ skipping 121 matching lines @@
 base::string16 ShellDelegateImpl::GetProductName() const {
   return base::string16();
 }
 
 gfx::Image ShellDelegateImpl::GetDeprecatedAcceleratorImage() const {
   return gfx::Image();
 }
 
 }  // namespace shell
 }  // namespace ash