Introduce toSVGAnimateElement(), and use it

Source/core/svg/SVGAnimateElement.cpp

Index: Source/core/svg/SVGAnimateElement.cpp
diff --git a/Source/core/svg/SVGAnimateElement.cpp b/Source/core/svg/SVGAnimateElement.cpp
index 0f404eb7d9fddea2d807dba16e38deafc9e00e4c..e3af159a79dc5158f9ba05c26d94f72ae63a365d 100644
--- a/Source/core/svg/SVGAnimateElement.cpp
+++ b/Source/core/svg/SVGAnimateElement.cpp
@@ -111,12 +111,7 @@ void SVGAnimateElement::calculateAnimatedValue(float percentage, unsigned repeat
     ASSERT(m_fromType->type() == m_animatedPropertyType);
     ASSERT(m_toType);
 
-    ASSERT(resultElement->hasTagName(SVGNames::animateTag)
-        || resultElement->hasTagName(SVGNames::animateColorTag)
-        || resultElement->hasTagName(SVGNames::animateTransformTag)
-        || resultElement->hasTagName(SVGNames::setTag));
-
-    SVGAnimateElement* resultAnimationElement = static_cast<SVGAnimateElement*>(resultElement);
+    SVGAnimateElement* resultAnimationElement = toSVGAnimateElement(resultElement);
     ASSERT(resultAnimationElement->m_animatedType);
     ASSERT(resultAnimationElement->m_animatedPropertyType == m_animatedPropertyType);