Introduce toSVGFEDiffuseLightingElement(), and use it

Source/core/svg/SVGFELightElement.cpp

Index: Source/core/svg/SVGFELightElement.cpp
diff --git a/Source/core/svg/SVGFELightElement.cpp b/Source/core/svg/SVGFELightElement.cpp
index 9cb96d9312cff15da356ef29dbac9f39987160e9..045653c8e9e37824f2562cb21c87b0f1d4c84ae6 100644
--- a/Source/core/svg/SVGFELightElement.cpp
+++ b/Source/core/svg/SVGFELightElement.cpp
@@ -190,8 +190,7 @@ void SVGFELightElement::svgAttributeChanged(const QualifiedName& attrName)
             return;
 
         if (parent->hasTagName(SVGNames::feDiffuseLightingTag)) {
-            SVGFEDiffuseLightingElement* diffuseLighting = static_cast<SVGFEDiffuseLightingElement*>(parent);
-            diffuseLighting->lightElementAttributeChanged(this, attrName);
+            toSVGFEDiffuseLightingElement(parent)->lightElementAttributeChanged(this, attrName);
             return;
         } else if (parent->hasTagName(SVGNames::feSpecularLightingTag)) {
             SVGFESpecularLightingElement* specularLighting = static_cast<SVGFESpecularLightingElement*>(parent);