Add toRenderSVGResourceRadialGradient() fuction, 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 ea6afb622bc8d30daae96c4cfadf2e884125f7e1..ada7f29ded6b76071701c51121b78156702ec558 100644
--- a/Source/core/rendering/svg/SVGRenderTreeAsText.cpp
+++ b/Source/core/rendering/svg/SVGRenderTreeAsText.cpp
@@ -541,7 +541,7 @@ void writeSVGResourceContainer(TextStream& ts, const RenderObject& object, int i
 
         ts << " [start=" << gradient->startPoint(attributes) << "] [end=" << gradient->endPoint(attributes) << "]\n";
     }  else if (resource->resourceType() == RadialGradientResourceType) {
-        RenderSVGResourceRadialGradient* gradient = static_cast<RenderSVGResourceRadialGradient*>(resource);
+        RenderSVGResourceRadialGradient* gradient = toRenderSVGResourceRadialGradient(resource);
 
         // 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()