Use new is*Element() helper functions more in core/ code

Source/core/clipboard/Clipboard.cpp

 /*
  * Copyright (C) 2006, 2007, 2008 Apple Inc. All rights reserved.
  *
  * 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
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 203 matching lines @@
 void Clipboard::setDragImage(Element* image, int x, int y, ExceptionState& exceptionState)
 {
     if (!isForDragAndDrop())
         return;
 
     if (!image) {
         exceptionState.throwTypeError("setDragImage: Invalid first argument");
         return;
     }
     IntPoint location(x, y);
-    if (image->hasTagName(HTMLNames::imgTag) && !image->inDocument())
-        setDragImageResource(toHTMLImageElement(image)->cachedImage(), location);
+    if (isHTMLImageElement(*image) && !image->inDocument())
+        setDragImageResource(toHTMLImageElement(*image).cachedImage(), location);
     else
         setDragImageElement(image, location);
 }
 
 void Clipboard::setDragImageResource(ImageResource* img, const IntPoint& loc)
 {
     setDragImage(img, 0, loc);
 }
 
 void Clipboard::setDragImageElement(Node* node, const IntPoint& loc)
@@ skipping 285 matching lines @@
         return String("copy");
     }
 }
 
 void Clipboard::trace(Visitor* visitor)
 {
     visitor->trace(m_dataObject);
 }
 
 } // namespace WebCore