Introduce toSVGLinearGradientElement(), 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..2106e01e81a285c540f4ce7dba7a6789b6c1ec47 100644
--- a/Source/core/rendering/svg/SVGRenderTreeAsText.cpp
+++ b/Source/core/rendering/svg/SVGRenderTreeAsText.cpp
@@ -539,10 +539,8 @@ 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()
-        SVGLinearGradientElement* linearGradientElement = static_cast<SVGLinearGradientElement*>(gradient->node());
-
         LinearGradientAttributes attributes;
-        linearGradientElement->collectGradientAttributes(attributes);
+        toSVGLinearGradientElement(gradient->node())->collectGradientAttributes(attributes);
         writeCommonGradientProperties(ts, attributes.spreadMethod(), attributes.gradientTransform(), attributes.gradientUnits());
 
         ts << " [start=" << gradient->startPoint(attributes) << "] [end=" << gradient->endPoint(attributes) << "]\n";