Implement a basic UI flow for cryptohome encryption migration.

chrome/browser/chromeos/login/screens/encryption_migration_screen.h

 // Copyright 2017 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 CHROME_BROWSER_CHROMEOS_LOGIN_SCREENS_ENCRYPTION_MIGRATION_SCREEN_H_
 #define CHROME_BROWSER_CHROMEOS_LOGIN_SCREENS_ENCRYPTION_MIGRATION_SCREEN_H_
 
 #include "chrome/browser/chromeos/login/screens/base_screen.h"
 #include "chrome/browser/chromeos/login/screens/encryption_migration_screen_view.h"
 
 namespace chromeos {
 
+class UserContext;
+
 class EncryptionMigrationScreen
     : public BaseScreen,
       public EncryptionMigrationScreenView::Delegate {
  public:
   EncryptionMigrationScreen(BaseScreenDelegate* base_screen_delegate,
                             EncryptionMigrationScreenView* view);
   ~EncryptionMigrationScreen() override;
 
-  // BaseScreen:
+  // BaseScreen implementation:
   void Show() override;
   void Hide() override;
 
-  // EncryptionMigrationScreenView::Delegate:
+  // EncryptionMigrationScreenView::Delegate implementation:
   void OnExit() override;
   void OnViewDestroyed(EncryptionMigrationScreenView* view) override;
 
+  // Sets the UserContext for a user whose cryptohome should be migrated.
+  void SetUserContext(const UserContext& user_context);
+
  private:
   EncryptionMigrationScreenView* view_;
 
   DISALLOW_COPY_AND_ASSIGN(EncryptionMigrationScreen);
 };
 
 }  // namespace chromeos
 
 #endif  // CHROME_BROWSER_CHROMEOS_LOGIN_SCREENS_ENCRYPTION_MIGRATION_SCREEN_H_