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

Source/core/html/HTMLVideoElement.h

Index: Source/core/html/HTMLVideoElement.h
diff --git a/Source/core/html/HTMLVideoElement.h b/Source/core/html/HTMLVideoElement.h
index 54a56ea19e21c83c4b8e10e492b321be84e5dd8a..a33183336183c04475f1097ba31ac31f8b2aeb09 100644
--- a/Source/core/html/HTMLVideoElement.h
+++ b/Source/core/html/HTMLVideoElement.h
@@ -71,7 +71,7 @@ public:
     // 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;
 
@@ -99,10 +99,12 @@ private:
     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;