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

Source/core/html/HTMLMediaElement.h

Index: Source/core/html/HTMLMediaElement.h
diff --git a/Source/core/html/HTMLMediaElement.h b/Source/core/html/HTMLMediaElement.h
index ca381547f012a480c77082af3fadf6a7b1f44aaf..215cb19eee1108da50b2908d8af0a92d3493e646 100644
--- a/Source/core/html/HTMLMediaElement.h
+++ b/Source/core/html/HTMLMediaElement.h
@@ -277,9 +277,7 @@ protected:
     void didMoveToNewDocument(Document& oldDocument) override;
     virtual KURL posterImageURL() const { return KURL(); }
 
-    enum DisplayMode { Unknown, Poster, Video };
-    DisplayMode displayMode() const { return m_displayMode; }
-    virtual void setDisplayMode(DisplayMode mode) { m_displayMode = mode; }
+    virtual void setShowPoster(bool showPoster) { }
 
     void setControllerInternal(MediaController*);
 
@@ -308,7 +306,7 @@ private:
     // ActiveDOMObject functions.
     void stop() final;
 
-    virtual void updateDisplayState() { }
+    virtual void updateDisplayState(bool forceUpdate) { }
 
     void setReadyState(ReadyState);
     void setNetworkState(WebMediaPlayer::NetworkState);
@@ -503,8 +501,6 @@ private:
     OwnPtr<WebMediaPlayer> m_webMediaPlayer;
     WebLayer* m_webLayer;
 
-    DisplayMode m_displayMode;
-
     RefPtrWillBeMember<HTMLMediaSource> m_mediaSource;
 
     // Cached time value. Only valid when ready state is HAVE_METADATA or