Return operation name in prefetch request callback and add internal page hookup

chrome/browser/resources/offline_pages/offline_internals.css

 /* Copyright 2016 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. */
 
 h1 {
   color: rgb(74, 142, 230);
   margin: 0;
   padding: 0;
 }
 
@@ skipping 32 matching lines @@
   padding: 0;
 }
 
 li:nth-child(2n) {
   background-color: lavender;
 }
 
 #current-status {
   font-size: 15px;
 }
+
+.dump {
+  font-family: monospace;
+  white-space: pre-wrap;
+}