Introduce toSVGUseElement(), use it

Source/core/rendering/svg/RenderSVGTransformableContainer.cpp

Index: Source/core/rendering/svg/RenderSVGTransformableContainer.cpp
diff --git a/Source/core/rendering/svg/RenderSVGTransformableContainer.cpp b/Source/core/rendering/svg/RenderSVGTransformableContainer.cpp
index e03be53828128efa9e8066eb4f32996a9fb10900..d53d89206fa32926bfd3b8c266a5368b3532fd7a 100644
--- a/Source/core/rendering/svg/RenderSVGTransformableContainer.cpp
+++ b/Source/core/rendering/svg/RenderSVGTransformableContainer.cpp
@@ -46,11 +46,11 @@ bool RenderSVGTransformableContainer::calculateLocalTransform()
     // need to respect the translations induced by their corresponding use elements x/y attributes.
     SVGUseElement* useElement = 0;
     if (element->hasTagName(SVGNames::useTag))
-        useElement = static_cast<SVGUseElement*>(element);
+        useElement = toSVGUseElement(element);
     else if (element->isInShadowTree() && element->hasTagName(SVGNames::gTag)) {
         SVGElement* correspondingElement = element->correspondingElement();
         if (correspondingElement && correspondingElement->hasTagName(SVGNames::useTag))
-            useElement = static_cast<SVGUseElement*>(correspondingElement);
+            useElement = toSVGUseElement(correspondingElement);
     }
 
     if (useElement) {