Add toSVGFontFaceUri|NameElement(), and use them.

Source/core/svg/SVGFontFaceSrcElement.cpp

Index: Source/core/svg/SVGFontFaceSrcElement.cpp
diff --git a/Source/core/svg/SVGFontFaceSrcElement.cpp b/Source/core/svg/SVGFontFaceSrcElement.cpp
index 849a2a4ba6c16e0c80fe09da1f5b9dc8aba5b132..e5ba1db9634660174760d1da7a8634cfeefddac9 100644
--- a/Source/core/svg/SVGFontFaceSrcElement.cpp
+++ b/Source/core/svg/SVGFontFaceSrcElement.cpp
@@ -51,9 +51,10 @@ PassRefPtr<CSSValueList> SVGFontFaceSrcElement::srcValue() const
     for (Node* child = firstChild(); child; child = child->nextSibling()) {
         RefPtr<CSSFontFaceSrcValue> srcValue;
         if (child->hasTagName(font_face_uriTag))
-            srcValue = static_cast<SVGFontFaceUriElement*>(child)->srcValue();
+            srcValue = toSVGFontFaceUriElement(child)->srcValue();
         else if (child->hasTagName(font_face_nameTag))
-            srcValue = static_cast<SVGFontFaceNameElement*>(child)->srcValue();
+            srcValue = toSVGFontFaceNameElement(child)->srcValue();
+
         if (srcValue && srcValue->resource().length())
             list->append(srcValue);
     }