cros: Prevent casual shut down and power save during migration.

chrome/browser/resources/chromeos/login/screen_encryption_migration.js

Index: chrome/browser/resources/chromeos/login/screen_encryption_migration.js
diff --git a/chrome/browser/resources/chromeos/login/screen_encryption_migration.js b/chrome/browser/resources/chromeos/login/screen_encryption_migration.js
index 70740d3483ee3d495879d83203c29d1ee8d5b71a..ae2ae6c2457db32c76b0a0fa3fa2638c76558467 100644
--- a/chrome/browser/resources/chromeos/login/screen_encryption_migration.js
+++ b/chrome/browser/resources/chromeos/login/screen_encryption_migration.js
@@ -48,10 +48,15 @@ login.createScreen('EncryptionMigrationScreen', 'encryption-migration',
     /**
      * Updates the migration screen by specifying a state which corresponds to
      * a sub step in the migration process.
-     * @param {number} state The UI state to identify a sub step in migration.
+     * @param {EncryptionMigrationUIState} state The UI state to identify a sub
+     *     step in migration.
      */
     setUIState: function(state) {
       $('encryption-migration-element').uiState = state;
+
+      // Hide "Shut down" button during migration.
+      $('login-header-bar').showShutdownButton =
+          state != EncryptionMigrationUIState.MIGRATING;
     },
 
     /**