Add toSVGMissingGlyphElement(), and use it.

Source/core/svg/SVGMissingGlyphElement.h

Index: Source/core/svg/SVGMissingGlyphElement.h
diff --git a/Source/core/svg/SVGMissingGlyphElement.h b/Source/core/svg/SVGMissingGlyphElement.h
index 9dfaf3ffd5db62eb6d76d67538ff1bfa16aac5dc..9437431ad913754ffd4464f37d52572eb61edf7f 100644
--- a/Source/core/svg/SVGMissingGlyphElement.h
+++ b/Source/core/svg/SVGMissingGlyphElement.h
@@ -21,6 +21,7 @@
 #define SVGMissingGlyphElement_h
 
 #if ENABLE(SVG_FONTS)
+#include "SVGNames.h"
 #include "core/svg/SVGElement.h"
 
 namespace WebCore {
@@ -35,6 +36,12 @@ private:
     virtual bool rendererIsNeeded(const NodeRenderingContext&) { return false; }
 };
 
+inline SVGMissingGlyphElement* toSVGMissingGlyphElement(Node* node)
+{
+    ASSERT_WITH_SECURITY_IMPLICATION(!node || node->hasTagName(SVGNames::missing_glyphTag));
+    return static_cast<SVGMissingGlyphElement*>(node);
+}
+
 } // namespace WebCore
 
 #endif // ENABLE(SVG_FONTS)