Update poster code as per https://html.spec.whatwg.org/#show-poster-flag

Source/core/html/HTMLVideoElement.cpp

 /*
  * Copyright (C) 2007, 2008, 2009, 2010 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 34 matching lines @@
 #include "platform/graphics/gpu/Extensions3DUtil.h"
 #include "public/platform/WebCanvas.h"
 #include "public/platform/WebGraphicsContext3D.h"
 
 namespace blink {
 
 using namespace HTMLNames;
 
 inline HTMLVideoElement::HTMLVideoElement(Document& document)
     : HTMLMediaElement(videoTag, document)
+    , m_displayMode(Unknown)
 {
     if (document.settings())
         m_defaultPosterURL = AtomicString(document.settings()->defaultVideoPosterURL());
 }
 
 PassRefPtrWillBeRawPtr<HTMLVideoElement> HTMLVideoElement::create(Document& document)
 {
     RefPtrWillBeRawPtr<HTMLVideoElement> video = adoptRefWillBeNoop(new HTMLVideoElement(document));
     video->ensureUserAgentShadowRoot();
     video->suspendIfNeeded();
@@ skipping 47 matching lines @@
     return HTMLMediaElement::isPresentationAttribute(name);
 }
 
 void HTMLVideoElement::parseAttribute(const QualifiedName& name, const AtomicString& value)
 {
     if (name == posterAttr) {
         // In case the poster attribute is set after playback, don't update the
         // display state, post playback the correct state will be picked up.
         if (displayMode() < Video || !hasAvailableVideoFrame()) {
             // Force a poster recalc by setting m_displayMode to Unknown directly before calling updateDisplayState.
-            HTMLMediaElement::setDisplayMode(Unknown);
+            m_displayMode = Unknown;
             updateDisplayState();
         }
         if (!posterImageURL().isEmpty()) {
             if (!m_imageLoader)
                 m_imageLoader = HTMLImageLoader::create(this);
             m_imageLoader->updateFromElement(ImageLoader::UpdateIgnorePreviousError);
         } else {
             if (layoutObject())
                 toLayoutImage(layoutObject())->imageResource()->setImageResource(0);
         }
@@ skipping 39 matching lines @@
 
     if (!poster.isEmpty()) {
         // We have a poster path, but only show it until the user triggers display by playing or seeking and the
         // media engine has something to display.
         // Don't show the poster if there is a seek operation or
         // the video has restarted because of loop attribute
         if (mode == Video && oldMode == Poster && !hasAvailableVideoFrame())
             return;
     }
 
-    HTMLMediaElement::setDisplayMode(mode);
+    m_displayMode = mode;
 
     if (layoutObject() && displayMode() != oldMode)
         layoutObject()->updateFromElement();
 }
 
 void HTMLVideoElement::updateDisplayState()
 {
     if (posterImageURL().isEmpty())
         setDisplayMode(Video);
     else if (displayMode() < Poster)
@@ skipping 126 matching lines @@
 {
     return !isMediaDataCORSSameOrigin(destinationSecurityOrigin);
 }
 
 FloatSize HTMLVideoElement::elementSize() const
 {
     return FloatSize(videoWidth(), videoHeight());
 }
 
 } // namespace blink