Implement a basic UI flow for cryptohome encryption migration.

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

 // 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.
 
 #include "chrome/browser/chromeos/login/screens/encryption_migration_screen.h"
 
 #include "base/logging.h"
 #include "chrome/browser/chromeos/login/screens/base_screen_delegate.h"
 #include "chrome/browser/chromeos/login/wizard_controller.h"
 
@@ skipping 27 matching lines @@
 void EncryptionMigrationScreen::OnExit() {
   Finish(ScreenExitCode::ENCRYPTION_MIGRATION_FINISHED);
 }
 
 void EncryptionMigrationScreen::OnViewDestroyed(
     EncryptionMigrationScreenView* view) {
   if (view_ == view)
     view_ = nullptr;
 }
 
+void EncryptionMigrationScreen::SetUserContext(
+    const UserContext& user_context) {
+  DCHECK(view_);
+  view_->SetUserContext(user_context);
+}
+
 }  // namespace chromeos