Add isHTMLPlugInElement() helpers for consistency with other HTML elements

Source/core/page/DragController.cpp

 /*
  * Copyright (C) 2007, 2009, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2008 Google Inc.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 536 matching lines @@
         return false;
 
     result = m_page->mainFrame()->eventHandler().hitTestResultAtPoint(point);
 
     if (!result.innerNonSharedNode())
         return false;
 
     if (dragData->containsFiles() && asFileInput(result.innerNonSharedNode()))
         return true;
 
-    if (result.innerNonSharedNode()->isPluginElement()) {
+    if (isHTMLPlugInElement(*result.innerNonSharedNode())) {
         HTMLPlugInElement* plugin = toHTMLPlugInElement(result.innerNonSharedNode());
         if (!plugin->canProcessDrag() && !result.innerNonSharedNode()->rendererIsEditable())
             return false;
     } else if (!result.innerNonSharedNode()->rendererIsEditable())
         return false;
 
     if (m_didInitiateDrag && m_documentUnderMouse == m_dragInitiator && result.isSelected())
         return false;
 
     return true;
@@ skipping 390 matching lines @@
 #else
     return keyState & PlatformEvent::CtrlKey;
 #endif
 }
 
 void DragController::cleanupAfterSystemDrag()
 {
 }
 
 } // namespace WebCore