Implement a basic UI flow for cryptohome encryption migration.

chrome/browser/chromeos/login/existing_user_controller.h

Index: chrome/browser/chromeos/login/existing_user_controller.h
diff --git a/chrome/browser/chromeos/login/existing_user_controller.h b/chrome/browser/chromeos/login/existing_user_controller.h
index fc6f551564eb5353be2a258d2f1b8626931dfedb..bf392b33db3956e56a2cb8b055079a304f2ffb98 100644
--- a/chrome/browser/chromeos/login/existing_user_controller.h
+++ b/chrome/browser/chromeos/login/existing_user_controller.h
@@ -163,7 +163,7 @@ class ExistingUserController
   void OnAuthSuccess(const UserContext& user_context) override;
   void OnOffTheRecordAuthSuccess() override;
   void OnPasswordChangeDetected() override;
-  void OnOldEncryptionDetected() override;
+  void OnOldEncryptionDetected(const UserContext& user_context) override;
   void WhiteListCheckFailed(const std::string& email) override;
   void PolicyLoadFailed() override;
   void SetAuthFlowOffline(bool offline) override;
@@ -208,7 +208,7 @@ class ExistingUserController
   void ShowKioskAutolaunchScreen();
 
   // Shows "filesystem encryption migration" screen.
-  void ShowEncryptionMigrationScreen();
+  void ShowEncryptionMigrationScreen(const UserContext& user_context);
 
   // Shows "critical TPM error" screen.
   void ShowTPMError();