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

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

 <!-- 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. -->
 
 <link rel="import" href="chrome://resources/cr_elements/icons.html">
 <link rel="import" href="chrome://resources/html/i18n_behavior.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-flex-layout/classes/iron-flex-layout.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-progress/paper-progress.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-styles/color.html">
 <link rel="import" href="chrome://oobe/custom_elements.html">
@@ skipping 72 matching lines @@
           <div class="subtitle">$i18n{migrationFailedSubtitle}</div>
         </div>
         <div class="footer"><div>$i18n{migrationFailedMessage}</div></div>
         <div class="bottom-buttons flex layout horizontal">
           <div class="flex"></div>
 <if expr="_google_chrome">
           <oobe-text-button border on-tap="onReportAnIssue_">
             <div>$i18n{migrationButtonReportAnIssue}</div>
           </oobe-text-button>
 </if>
-          <oobe-text-button inverse on-tap="onRestart_">
+          <oobe-text-button inverse on-tap="onRestartOnFailure_">
             <div>$i18n{migrationButtonRestart}</div>
           </oobe-text-button>
         </div>
       </oobe-dialog>
     </template>
     <template is="dom-if" if="[[isNotEnoughSpace_(uiState)]]">
       <oobe-dialog tabindex="0" has-buttons>
         <iron-icon class="oobe-icon chrome-logo"></iron-icon>
         <div class="header">
           <h1 class="title">$i18n{migrationReadyTitle}</h1>
           <div class="subtitle">$i18n{migrationReadyDescription}</div>
         </div>
         <div class="footer layout vertical" aria-live="polite">
           <div class="warning">$i18n{migrationNospaceWarningLabel}</div>
           <div>$i18n{migrationAskFreeSpaceMessage}</div>
           <div>[[computeAvailableSpaceLabel_(availableSpaceInString)]]</div>
           <div>[[computeNecessarySpaceLabel_(necessarySpaceInString)]]</div>
         </div>
         <div class="bottom-buttons flex layout horizontal">
           <div class="flex"></div>
           <template is="dom-if" if="[[!isResuming]]">
             <oobe-text-button inverse on-tap="onSkip_">
               <div>$i18n{migrationButtonContinue}</div>
             </oobe-text-button>
           </template>
           <template is="dom-if" if="[[isResuming]]">
             <oobe-text-button inverse on-tap="onRestart_">

[xiyuan, 2017/06/01 15:27:16]

nit: onRestart_ -> onRestartOnLowStorage_ similar to onRestartOnFailure_. And
update all names along the way until the C++ handler.

i.e.
method name:
  onRestart_ -> onRestartOnLowStorage_
event name:
  'restart' -> 'restart-on-low-storage'
chrome.send message:
  'requestRestart' -> 'requestRestartOnLowStorage'
c++ handler
  HandleRequestRestart -> HandleRequestRestartOnLowStorage

This would make it easier to follow the code.

[fukino, 2017/06/02 03:39:36]

Done
I'm sorry I was too lazy...!

               <div>$i18n{migrationButtonRestart}</div>
             </oobe-text-button>
           </template>
         </div>
       </oobe-dialog>
     </template>
   </template>
 </dom-module>