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 e64a10fd0aa63fa70bd7c1a0daedc0f29a5d553a..640dce8a006a5a93a0cd6604d92a2ebdac9626c6 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_showPoster(true)
 {
     if (document.settings())
         m_defaultPosterURL = AtomicString(document.settings()->defaultVideoPosterURL());
@@ -85,7 +86,7 @@ void HTMLVideoElement::attach(const AttachContext& context)
 {
     HTMLMediaElement::attach(context);
 
-    updateDisplayState();
+    updateDisplayState(true);
     if (shouldDisplayPosterImage()) {
         if (!m_imageLoader)
             m_imageLoader = HTMLImageLoader::create(this);
@@ -117,10 +118,9 @@ void HTMLVideoElement::parseAttribute(const QualifiedName& name, const AtomicStr
     if (name == posterAttr) {
         // In case the poster attribute is set after playback, don't update the
         // 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);
-            updateDisplayState();
+        if (m_showPoster || !hasAvailableVideoFrame()) {
+            m_showPoster = true;
+            updateDisplayState(true);
         }
         if (!posterImageURL().isEmpty()) {
             if (!m_imageLoader)
@@ -165,9 +165,8 @@ const AtomicString HTMLVideoElement::imageSourceURL() const
     return m_defaultPosterURL;
 }
 
-void HTMLVideoElement::setDisplayMode(DisplayMode mode)
+void HTMLVideoElement::setShowPoster(bool showPoster)
 {
-    DisplayMode oldMode = displayMode();
     KURL poster = posterImageURL();
 
     if (!poster.isEmpty()) {
@@ -175,22 +174,26 @@ void HTMLVideoElement::setDisplayMode(DisplayMode mode)
         // media engine has something to display.
         // Don't show the poster if there is a seek operation or
         // the video has restarted because of loop attribute
-        if (mode == Video && oldMode == Poster && !hasAvailableVideoFrame())
+        if (!showPoster && m_showPoster && !hasAvailableVideoFrame())
             return;
     }
 
-    HTMLMediaElement::setDisplayMode(mode);
-
-    if (layoutObject() && displayMode() != oldMode)
-        layoutObject()->updateFromElement();
+    if (showPoster != m_showPoster) {
+        m_showPoster = showPoster;
+        if (layoutObject())
+            layoutObject()->updateFromElement();
+    }
 }
 
-void HTMLVideoElement::updateDisplayState()
+void HTMLVideoElement::updateDisplayState(bool forceUpdate)
 {
-    if (posterImageURL().isEmpty())
-        setDisplayMode(Video);
-    else if (displayMode() < Poster)
-        setDisplayMode(Poster);
+    if (posterImageURL().isEmpty()) {
+        setShowPoster(false);
+    } else if (m_showPoster) {
+        if (forceUpdate)
+            m_showPoster = false;
+        setShowPoster(true);
+    }
 }
 
 void HTMLVideoElement::paintCurrentFrame(SkCanvas* canvas, const IntRect& destRect, const SkPaint* paint) const