Introduce toSVGFESpecularLightingElement(), 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..2ac7e0931abaf801d3ac828af2dcd8f75b08ff33 100644
--- a/Source/core/svg/SVGFELightElement.cpp
+++ b/Source/core/svg/SVGFELightElement.cpp
@@ -194,8 +194,7 @@ void SVGFELightElement::svgAttributeChanged(const QualifiedName& attrName)
             diffuseLighting->lightElementAttributeChanged(this, attrName);
             return;
         } else if (parent->hasTagName(SVGNames::feSpecularLightingTag)) {
-            SVGFESpecularLightingElement* specularLighting = static_cast<SVGFESpecularLightingElement*>(parent);
-            specularLighting->lightElementAttributeChanged(this, attrName);
+            toSVGFESpecularLightingElement(parent)->lightElementAttributeChanged(this, attrName);
             return;
         }
     }