cros: Add UMA metrics in migration UI to get more information from migration failures.

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

 // 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.
 
 /**
  * @fileoverview Polymer element for displaying encryption migration screen.
  */
 
 /**
  * Enum for the UI states corresponding to sub steps inside migration screen.
@@ skipping 198 matching lines @@
    * @private
    */
   onSkip_: function() {
     this.fire('skip');
   },
 
   /**
    * Handles tap on RESTART button.
    * @private
    */
-  onRestart_: function() {
-    this.fire('restart');
+  onRestartOnLowStorage_: function() {
+    this.fire('restart-on-low-storage');
   },
 
   /**
+   * Handles tap on RESTART button on the migration failure screen.
+   * @private
+   */
+  onRestartOnFailure_: function() {
+    this.fire('restart-on-failure');
+  },
+
+  /**
    * Handles tap on REPORT AN ISSUE button.
    * @private
    */
   onReportAnIssue_: function() {
     this.fire('openFeedbackDialog');
   },
 });