Introduce toSVGFontFaceElement, and use it

Source/core/svg/SVGFontFaceElement.h

Index: Source/core/svg/SVGFontFaceElement.h
diff --git a/Source/core/svg/SVGFontFaceElement.h b/Source/core/svg/SVGFontFaceElement.h
index 844ac294a604e2f248ca33e45dd79f90ddc3878f..0da43df0660dc0dade71b06d4344141291c96e98 100644
--- a/Source/core/svg/SVGFontFaceElement.h
+++ b/Source/core/svg/SVGFontFaceElement.h
@@ -23,6 +23,7 @@
 #define SVGFontFaceElement_h
 
 #if ENABLE(SVG_FONTS)
+#include "SVGNames.h"
 #include "core/svg/SVGElement.h"
 
 namespace WebCore {
@@ -64,6 +65,12 @@ private:
     SVGFontElement* m_fontElement;
 };
 
+inline SVGFontFaceElement* toSVGFontFaceElement(Node* node)
+{
+    ASSERT_WITH_SECURITY_IMPLICATION(!node || node->hasTagName(SVGNames::font_faceTag));
+    return static_cast<SVGFontFaceElement*>(node);
+}
+
 } // namespace WebCore
 
 #endif // ENABLE(SVG_FONTS)