Make extension error overlay remotely keyboard accessible

chrome/browser/resources/extensions/extension_error.css

 /* Copyright 2013 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. */
 
 .extension-error-list-heading {
   align-content: flex-start;
   display: flex;
   flex-direction: row;
   justify-content: space-between;
   padding: 3px;
@@ skipping 52 matching lines @@
   margin-top: 0;
   overflow: hidden;
 }
 
 .extension-error-metadata {
   align-items: center;
   display: flex;
 }
 
 .extension-error-metadata > .error-delete-button {
+  -webkit-appearance: none;
   background: url(chrome://theme/IDR_CLOSE_DIALOG) center no-repeat;
+  border: none;
   height: 14px;
   opacity: 0.6;
   width: 14px;
 }
 
 .extension-error-metadata > .error-delete-button:hover {
   opacity: 0.8;
 }
 
 .extension-error-metadata > .error-delete-button:active {
@@ skipping 16 matching lines @@
   content: url(extension_error_severity_warning.png);
 }
 
 .extension-error-severity-fatal .extension-error-message {
   color: rgba(200, 50, 50, 255);
 }
 .extension-error-severity-fatal .extension-error-icon,
 .extension-error-fatal-icon {
   content: url(extension_error_severity_fatal.png);
 }