Introduce toSVGRadialGradientElement(), and use it

Source/core/rendering/svg/RenderSVGResourceGradient.cpp

Index: Source/core/rendering/svg/RenderSVGResourceGradient.cpp
diff --git a/Source/core/rendering/svg/RenderSVGResourceGradient.cpp b/Source/core/rendering/svg/RenderSVGResourceGradient.cpp
index a23c163efcc3d856016b0a2ec69be3cbf69476ec..51e6782f536cc6a1702ee3af58ef34bd017d0fd7 100644
--- a/Source/core/rendering/svg/RenderSVGResourceGradient.cpp
+++ b/Source/core/rendering/svg/RenderSVGResourceGradient.cpp
@@ -62,7 +62,7 @@ bool RenderSVGResourceGradient::applyResource(RenderObject* object, RenderStyle*
     // Otherwhise the call to collectGradientAttributes() in createTileImage(), may cause the SVG DOM property
     // synchronization to kick in, which causes removeAllClientsFromCache() to be called, which in turn deletes our
     // GradientData object! Leaving out the line below will cause svg/dynamic-updates/SVG*GradientElement-svgdom* to crash.
-    SVGGradientElement* gradientElement = static_cast<SVGGradientElement*>(node());
+    SVGGradientElement* gradientElement = toSVGGradientElement(node());
     if (!gradientElement)
         return false;