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

Source/core/clipboard/Clipboard.cpp

Index: Source/core/clipboard/Clipboard.cpp
diff --git a/Source/core/clipboard/Clipboard.cpp b/Source/core/clipboard/Clipboard.cpp
index 1e77906d7228bbf58e68cafc7101bbee629f7514..388efc53a989c63a659d57a49a6d9c8c7df5f6b1 100644
--- a/Source/core/clipboard/Clipboard.cpp
+++ b/Source/core/clipboard/Clipboard.cpp
@@ -221,8 +221,8 @@ void Clipboard::setDragImage(Element* image, int x, int y, ExceptionState& excep
         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);
 }