Do not use LayoutProgress for a UA shadow element of a PROGRESS element.

third_party/WebKit/Source/core/css/html.css

 /*
  * The default style sheet used to render HTML.
  *
  * Copyright (C) 2000 Lars Knoll (knoll@kde.org)
  * Copyright (C) 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
@@ skipping 879 matching lines @@
 progress {
     -webkit-appearance: progress-bar;
     box-sizing: border-box;
     display: inline-block;
     height: 1em;
     width: 10em;
     vertical-align: -0.2em;
 }
 
 progress::-webkit-progress-inner-element {
-    -webkit-appearance: inherit;
     box-sizing: inherit;
     -webkit-user-modify: read-only;
     height: 100%;
     width: 100%;
 }
 
 progress::-webkit-progress-bar {
     background-color: gray;
     height: 100%;
     width: 100%;
@@ skipping 207 matching lines @@
 @page {
     /* FIXME: Define the right default values for page properties. */
     size: auto;
     margin: auto;
     padding: 0px;
     border-width: 0px;
 }
 
 /* noscript is handled internally, as it depends on settings. */