Use isSVG*Element() helpers more in SVG code (Part 3)

Source/core/rendering/svg/RenderSVGGradientStop.cpp

 /*
  * Copyright (C) 2007 Eric Seidel <eric@webkit.org>
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 49 matching lines @@
 }
 
 void RenderSVGGradientStop::layout()
 {
     clearNeedsLayout();
 }
 
 SVGGradientElement* RenderSVGGradientStop::gradientElement() const
 {
     ContainerNode* parentNode = node()->parentNode();
-    if (parentNode->hasTagName(linearGradientTag) || parentNode->hasTagName(radialGradientTag))
-        return toSVGGradientElement(parentNode);
-    return 0;
+    ASSERT(parentNode);
+    return isSVGGradientElement(*parentNode) ? toSVGGradientElement(parentNode) : 0;
 }
 
 }