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

third_party/WebKit/Source/core/layout/LayoutProgress.cpp

 /*
  * Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies).
  *
  * 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.
  *
  * This library is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  * Library General Public License for more details.
  *
  * You should have received a copy of the GNU Library General Public License
  * along with this library; see the file COPYING.LIB.  If not, write to
  * the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
  * Boston, MA 02110-1301, USA.
  *
  */
 
 #include "core/layout/LayoutProgress.h"
 
 #include "core/html/HTMLProgressElement.h"
 #include "core/layout/LayoutTheme.h"
 #include "wtf/CurrentTime.h"
 #include "wtf/RefPtr.h"
 
 namespace blink {
 
-LayoutProgress::LayoutProgress(HTMLElement* element)
+LayoutProgress::LayoutProgress(HTMLProgressElement* element)
     : LayoutBlockFlow(element)
     , m_position(HTMLProgressElement::InvalidPosition)
     , m_animationStartTime(0)
     , m_animationRepeatInterval(0)
     , m_animationDuration(0)
     , m_animating(false)
     , m_animationTimer(this, &LayoutProgress::animationTimerFired)
 {
 }
 
@@ skipping 63 matching lines @@
     if (m_animating) {
         m_animationStartTime = currentTime();
         m_animationTimer.startOneShot(m_animationRepeatInterval, BLINK_FROM_HERE);
     } else {
         m_animationTimer.stop();
     }
 }
 
 HTMLProgressElement* LayoutProgress::progressElement() const
 {
-    if (!node())
-        return nullptr;
-
-    if (isHTMLProgressElement(*node()))
-        return toHTMLProgressElement(node());
-
-    ASSERT(node()->shadowHost());
-    return toHTMLProgressElement(node()->shadowHost());
+    return toHTMLProgressElement(node());
 }
 
 } // namespace blink