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

Source/core/plugins/PluginOcclusionSupport.cpp

 /*
  * Copyright (C) 2011 Google 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 161 matching lines @@
 
         const FrameView* frameView = toFrameView((*it).get());
         // Check to make sure we can get both the element and the RenderObject
         // for this FrameView, if we can't just move on to the next object.
         HTMLElement* element = frameView->frame().ownerElement();
         if (!element || !element->renderer())
             continue;
 
         RenderObject* iframeRenderer = element->renderer();
 
-        if (element->hasTagName(HTMLNames::iframeTag) && intersectsRect(iframeRenderer, frameRect)) {
+        if (isHTMLIFrameElement(*element) && intersectsRect(iframeRenderer, frameRect)) {
             getObjectStack(iframeRenderer, &iframeZstack);
             if (iframeIsAbovePlugin(iframeZstack, pluginZstack))
                 addToOcclusions(toRenderBox(iframeRenderer), occlusions);
         }
     }
 
     // Occlusions by top layer elements.
     // FIXME: There's no handling yet for the interaction between top layer and
     // iframes. For example, a plugin in the top layer will be occluded by an
     // iframe. And a plugin inside an iframe in the top layer won't be respected
     // as being in the top layer.
     const Element* ancestor = topLayerAncestor(element);
     Document* document = parentFrameView->frame().document();
     const Vector<RefPtr<Element> >& elements = document->topLayerElements();
     size_t start = ancestor ? elements.find(ancestor) + 1 : 0;
     for (size_t i = start; i < elements.size(); ++i)
         addTreeToOcclusions(elements[i]->renderer(), frameRect, occlusions);
 }
 
 } // namespace WebCore