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 53 matching lines @@
     // Statistics
     unsigned webkitDecodedFrameCount() const;
     unsigned webkitDroppedFrameCount() const;
 
     // Used by canvas to gain raw pixel access
     void paintCurrentFrame(SkCanvas*, const IntRect&, const SkPaint*) const;
 
     // Used by WebGL to do GPU-GPU textures copy if possible.
     bool copyVideoTextureToPlatformTexture(WebGraphicsContext3D*, Platform3DObject texture, GLenum internalFormat, GLenum type, bool premultiplyAlpha, bool flipY);
 
-    bool shouldDisplayPosterImage() const { return displayMode() == Poster; }
+    bool shouldDisplayPosterImage() const { return m_showPoster; }
 
     bool hasAvailableVideoFrame() const;
 
     KURL posterImageURL() const override;
 
     // CanvasImageSource implementation
     PassRefPtr<Image> getSourceImageForCanvas(SourceImageStatus*) const override;
     bool isVideoElement() const override { return true; }
     bool wouldTaintOrigin(SecurityOrigin*) const override;
     FloatSize elementSize() const override;
     const KURL& sourceURL() const override { return currentSrc(); }
 
     bool isHTMLVideoElement() const override { return true; }
 
 private:
     HTMLVideoElement(Document&);
 
     bool layoutObjectIsNeeded(const ComputedStyle&) override;
     LayoutObject* createLayoutObject(const ComputedStyle&) override;
     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 updateDisplayState(bool forceUpdate) override;
     void didMoveToNewDocument(Document& oldDocument) override;
-    void setDisplayMode(DisplayMode) override;
+    void setShowPoster(bool) override;
 
+    // whether to show poster or video frame
+    bool m_showPoster : 1;
     OwnPtrWillBeMember<HTMLImageLoader> m_imageLoader;
 
     AtomicString m_defaultPosterURL;
 };
 
 } // namespace blink
 
 #endif // HTMLVideoElement_h