Add isHTMLPlugInElement() helpers for consistency with other HTML elements

Source/core/html/parser/HTMLConstructionSite.cpp

Index: Source/core/html/parser/HTMLConstructionSite.cpp
diff --git a/Source/core/html/parser/HTMLConstructionSite.cpp b/Source/core/html/parser/HTMLConstructionSite.cpp
index 79feebf8653eab2938b837cee40257674a86b12f..e5e2efe67145ab056b18f4b28a1d6e567f18867c 100644
--- a/Source/core/html/parser/HTMLConstructionSite.cpp
+++ b/Source/core/html/parser/HTMLConstructionSite.cpp
@@ -38,6 +38,7 @@
 #include "core/frame/LocalFrame.h"
 #include "core/html/HTMLFormElement.h"
 #include "core/html/HTMLHtmlElement.h"
+#include "core/html/HTMLPlugInElement.h"
 #include "core/html/HTMLScriptElement.h"
 #include "core/html/HTMLTemplateElement.h"
 #include "core/html/parser/AtomicHTMLToken.h"
@@ -274,7 +275,7 @@ void HTMLConstructionSite::queueTask(const HTMLConstructionSiteTask& task)
 void HTMLConstructionSite::attachLater(ContainerNode* parent, PassRefPtr<Node> prpChild, bool selfClosing)
 {
     ASSERT(scriptingContentIsAllowed(m_parserContentPolicy) || !prpChild.get()->isElementNode() || !toScriptLoaderIfPossible(toElement(prpChild.get())));
-    ASSERT(pluginContentIsAllowed(m_parserContentPolicy) || !prpChild->isPluginElement());
+    ASSERT(pluginContentIsAllowed(m_parserContentPolicy) || !isHTMLPlugInElement(prpChild));
 
     HTMLConstructionSiteTask task(HTMLConstructionSiteTask::Insert);
     task.parent = parent;