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

Source/core/html/HTMLMediaElement.cpp

Index: Source/core/html/HTMLMediaElement.cpp
diff --git a/Source/core/html/HTMLMediaElement.cpp b/Source/core/html/HTMLMediaElement.cpp
index 608390e31002930cf2cd7594880257f0dc9ea7d3..5ecdff969b33b30244f1692e2c4a101c029f19aa 100644
--- a/Source/core/html/HTMLMediaElement.cpp
+++ b/Source/core/html/HTMLMediaElement.cpp
@@ -337,7 +337,6 @@ HTMLMediaElement::HTMLMediaElement(const QualifiedName& tagName, Document& docum
     , m_deferredLoadState(NotDeferred)
     , m_deferredLoadTimer(this, &HTMLMediaElement::deferredLoadTimerFired)
     , m_webLayer(nullptr)
-    , m_displayMode(Unknown)
     , m_cachedTime(std::numeric_limits<double>::quiet_NaN())
     , m_fragmentEndTime(std::numeric_limits<double>::quiet_NaN())
     , m_pendingActionFlags(0)
@@ -732,7 +731,7 @@ void HTMLMediaElement::prepareForLoad()
     m_haveFiredLoadedData = false;
     m_completelyLoaded = false;
     m_havePreparedToPlay = false;
-    m_displayMode = Unknown;
+    setDisplayMode(Unknown);
 
     // 1 - Abort any already-running instance of the resource selection algorithm for this element.
     m_loadState = WaitingForSource;