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

Source/core/html/HTMLMediaElement.h

 /*
  * Copyright (C) 2007, 2008, 2009, 2010, 2011, 2012, 2013 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 259 matching lines @@
 #endif
 
     void parseAttribute(const QualifiedName&, const AtomicString&) override;
     void finishParsingChildren() final;
     bool isURLAttribute(const Attribute&) const override;
     void attach(const AttachContext& = AttachContext()) override;
 
     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*);
 
 private:
     void resetMediaPlayerAndMediaSource();
 
     bool alwaysCreateUserAgentShadowRoot() const final { return true; }
     bool areAuthorShadowsAllowed() const final { return false; }
 
     bool supportsFocus() const final;
     bool isMouseFocusable() const final;
     bool layoutObjectIsNeeded(const ComputedStyle&) override;
     LayoutObject* createLayoutObject(const ComputedStyle&) override;
     InsertionNotificationRequest insertedInto(ContainerNode*) final;
     void didNotifySubtreeInsertionsToDocument() override;
     void removedFrom(ContainerNode*) final;
     void didRecalcStyle(StyleRecalcChange) final;
 
     bool canStartSelection() const override { return false; }
 
     void didBecomeFullscreenElement() final;
     void willStopBeingFullscreenElement() final;
     bool isInteractiveContent() const final;
     void defaultEventHandler(Event*) final;
 
     // ActiveDOMObject functions.
     void stop() final;
 
-    virtual void updateDisplayState() { }
+    virtual void updateDisplayState(bool forceUpdate) { }
 
     void setReadyState(ReadyState);
     void setNetworkState(WebMediaPlayer::NetworkState);
 
     // WebMediaPlayerClient implementation.
     void networkStateChanged() final;
     void readyStateChanged() final;
     void timeChanged() final;
     void repaint() final;
     void durationChanged() final;
@@ skipping 174 matching lines @@
         // The load is deferred, and waiting for the task to set the
         // delaying-the-load-event flag, after which the load will be executed.
         ExecuteOnStopDelayingLoadEventTask
     };
     DeferredLoadState m_deferredLoadState;
     Timer<HTMLMediaElement> m_deferredLoadTimer;
 
     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
     // higher, otherwise the current time is assumed to be zero.
     mutable double m_cachedTime;
 
     double m_fragmentEndTime;
 
     typedef unsigned PendingActionFlags;
     PendingActionFlags m_pendingActionFlags;
@@ skipping 106 matching lines @@
 inline bool isHTMLMediaElement(const HTMLElement& element)
 {
     return isHTMLAudioElement(element) || isHTMLVideoElement(element);
 }
 
 DEFINE_HTMLELEMENT_TYPE_CASTS_WITH_FUNCTION(HTMLMediaElement);
 
 } // namespace blink
 
 #endif // HTMLMediaElement_h