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

chrome/browser/ui/ash/chrome_shell_content_state_chromeos.cc

 // 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.
 
 #include "chrome/browser/ui/ash/chrome_shell_content_state.h"
 
 #include "ash/session/session_state_delegate.h"
 #include "ash/shell.h"
 #include "chrome/browser/chromeos/profiles/profile_helper.h"
 #include "chrome/browser/ui/ash/multi_user/multi_user_util.h"
 #include "chrome/browser/ui/ash/multi_user/multi_user_window_manager.h"
+#include "components/signin/core/account_id/account_id.h"
 #include "components/user_manager/user_manager.h"
 #include "content/public/browser/browser_context.h"
 
 content::BrowserContext* ChromeShellContentState::GetBrowserContextByIndex(
     ash::UserIndex index) {
   ash::SessionStateDelegate* session_state_delegate =
       ash::Shell::GetInstance()->session_state_delegate();
   DCHECK_LT(index, session_state_delegate->NumberOfLoggedInUsers());
   user_manager::User* user =
       user_manager::UserManager::Get()->GetLRULoggedInUsers()[index];
   CHECK(user);
   return chromeos::ProfileHelper::Get()->GetProfileByUser(user);
 }
 
 content::BrowserContext* ChromeShellContentState::GetBrowserContextForWindow(
     aura::Window* window) {
-  const std::string& user_id =
+  const AccountId& account_id =
       chrome::MultiUserWindowManager::GetInstance()->GetWindowOwner(window);
-  return user_id.empty() ? nullptr
-                         : multi_user_util::GetProfileFromUserID(user_id);
+  return account_id.is_valid()
+             ? multi_user_util::GetProfileFromAccountId(account_id)
+             : nullptr;
 }
 
 content::BrowserContext*
 ChromeShellContentState::GetUserPresentingBrowserContextForWindow(
     aura::Window* window) {
-  const std::string& user_id =
+  const AccountId& account_id =
       chrome::MultiUserWindowManager::GetInstance()->GetUserPresentingWindow(
           window);
-  return user_id.empty() ? nullptr
-                         : multi_user_util::GetProfileFromUserID(user_id);
+  return account_id.is_valid()
+             ? multi_user_util::GetProfileFromAccountId(account_id)
+             : nullptr;
 }