Introduce toSVGRadialGradientElement(), and use it

Source/core/rendering/svg/SVGRenderTreeAsText.cpp

Index: Source/core/rendering/svg/SVGRenderTreeAsText.cpp
diff --git a/Source/core/rendering/svg/SVGRenderTreeAsText.cpp b/Source/core/rendering/svg/SVGRenderTreeAsText.cpp
index 5838081a05813bfc4fec1e998c24edcc91d7a07a..f117b1df157b81408478e4d708844a8373e3cbd2 100644
--- a/Source/core/rendering/svg/SVGRenderTreeAsText.cpp
+++ b/Source/core/rendering/svg/SVGRenderTreeAsText.cpp
@@ -551,10 +551,8 @@ void writeSVGResourceContainer(TextStream& ts, const RenderObject& object, int i
 
         // Dump final results that are used for rendering. No use in asking SVGGradientElement for its gradientUnits(), as it may
         // link to other gradients using xlink:href, we need to build the full inheritance chain, aka. collectGradientProperties()
-        SVGRadialGradientElement* radialGradientElement = static_cast<SVGRadialGradientElement*>(gradient->node());
-
         RadialGradientAttributes attributes;
-        radialGradientElement->collectGradientAttributes(attributes);
+        toSVGRadialGradientElement(gradient->node())->collectGradientAttributes(attributes);
         writeCommonGradientProperties(ts, attributes.spreadMethod(), attributes.gradientTransform(), attributes.gradientUnits());
 
         FloatPoint focalPoint = gradient->focalPoint(attributes);