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

Source/core/rendering/RenderLayer.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@web.de>
  *   Randall Jesup <rjesup@wgate.com>
@@ skipping 2019 matching lines @@
                 rootRelativeBounds = calculateLayerBounds(paintingInfo.rootLayer, &offsetFromRoot, 0);
                 rootRelativeBoundsComputed = true;
             }
 
             context->clipPath(clipPath->path(rootRelativeBounds), clipPath->windRule());
         } else if (style->clipPath()->type() == ClipPathOperation::REFERENCE) {
             ReferenceClipPathOperation* referenceClipPathOperation = toReferenceClipPathOperation(style->clipPath());
             Document& document = renderer()->document();
             // FIXME: It doesn't work with forward or external SVG references (https://bugs.webkit.org/show_bug.cgi?id=90405)
             Element* element = document.getElementById(referenceClipPathOperation->fragment());
-            if (element && element->hasTagName(SVGNames::clipPathTag) && element->renderer()) {
+            if (isSVGClipPathElement(element) && element->renderer()) {
                 if (!rootRelativeBoundsComputed) {
                     rootRelativeBounds = calculateLayerBounds(paintingInfo.rootLayer, &offsetFromRoot, 0);
                     rootRelativeBoundsComputed = true;
                 }
 
                 resourceClipper = toRenderSVGResourceClipper(toRenderSVGResourceContainer(element->renderer()));
                 if (!resourceClipper->applyClippingToContext(renderer(), rootRelativeBounds,
                     paintingInfo.paintDirtyRect, context, clipperContext)) {
                     // No need to post-apply the clipper if this failed.
                     resourceClipper = 0;
@@ skipping 2013 matching lines @@
     }
 }
 
 void showLayerTree(const WebCore::RenderObject* renderer)
 {
     if (!renderer)
         return;
     showLayerTree(renderer->enclosingLayer());
 }
 #endif