Add <menuitem>, new HTMLMenuElement IDL attributes.

Source/core/rendering/RenderTheme.cpp

Index: Source/core/rendering/RenderTheme.cpp
diff --git a/Source/core/rendering/RenderTheme.cpp b/Source/core/rendering/RenderTheme.cpp
index 86408f068552457e2c777ab373da5205930d927d..d814b12f4bca4a511bc4b982a7993170fe71522b 100644
--- a/Source/core/rendering/RenderTheme.cpp
+++ b/Source/core/rendering/RenderTheme.cpp
@@ -419,6 +419,10 @@ String RenderTheme::extraDefaultStyleSheet()
         runtimeCSS.appendLiteral("dialog::backdrop { position: fixed; top: 0; right: 0; bottom: 0; left: 0; background: rgba(0,0,0,0.1); }");
     }
 
+    if (RuntimeEnabledFeatures::contextMenuEnabled()) {
+        runtimeCSS.appendLiteral("menu[type=\"popup\" i] { display: none; }");
+    }
+
     return runtimeCSS.toString();
 }