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

Source/core/html/HTMLVideoElement.h

 /*
  * 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 83 matching lines @@
     void attach(const AttachContext& = AttachContext()) override;
     void parseAttribute(const QualifiedName&, const AtomicString&) override;
     bool isPresentationAttribute(const QualifiedName&) const override;
     void collectStyleForPresentationAttribute(const QualifiedName&, const AtomicString&, MutableStylePropertySet*) override;
     bool hasVideo() const override { return webMediaPlayer() && webMediaPlayer()->hasVideo(); }
     bool isURLAttribute(const Attribute&) const override;
     const AtomicString imageSourceURL() const override;
 
     void updateDisplayState() override;
     void didMoveToNewDocument(Document& oldDocument) override;
+    DisplayMode displayMode() const override { return m_displayMode; }
     void setDisplayMode(DisplayMode) override;
 
+    DisplayMode m_displayMode;
     OwnPtrWillBeMember<HTMLImageLoader> m_imageLoader;
 
     AtomicString m_defaultPosterURL;
 };
 
 } // namespace blink
 
 #endif // HTMLVideoElement_h