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

Source/core/html/HTMLVideoElement.cpp

Index: Source/core/html/HTMLVideoElement.cpp
diff --git a/Source/core/html/HTMLVideoElement.cpp b/Source/core/html/HTMLVideoElement.cpp
index ed14340174e9d5733e4e79dfb76c4fa9e021d141..90e8df689aaf0506caddf99596c4e6201714b72f 100644
--- a/Source/core/html/HTMLVideoElement.cpp
+++ b/Source/core/html/HTMLVideoElement.cpp
@@ -52,6 +52,7 @@ using namespace HTMLNames;
 
 inline HTMLVideoElement::HTMLVideoElement(Document& document)
     : HTMLMediaElement(videoTag, document)
+    , m_displayMode(Unknown)
 {
     if (document.settings())
         m_defaultPosterURL = AtomicString(document.settings()->defaultVideoPosterURL());
@@ -119,7 +120,7 @@ void HTMLVideoElement::parseAttribute(const QualifiedName& name, const AtomicStr
         // display state, post playback the correct state will be picked up.
         if (displayMode() < Video || !hasAvailableVideoFrame()) {
             // Force a poster recalc by setting m_displayMode to Unknown directly before calling updateDisplayState.
-            HTMLMediaElement::setDisplayMode(Unknown);
+            m_displayMode = Unknown;
             updateDisplayState();
         }
         if (!posterImageURL().isEmpty()) {
@@ -179,7 +180,7 @@ void HTMLVideoElement::setDisplayMode(DisplayMode mode)
             return;
     }
 
-    HTMLMediaElement::setDisplayMode(mode);
+    m_displayMode = mode;
 
     if (layoutObject() && displayMode() != oldMode)
         layoutObject()->updateFromElement();