Add toSVGGlyphRefElement(), and use it as much as possible

Source/core/svg/SVGGlyphRefElement.h

Index: Source/core/svg/SVGGlyphRefElement.h
diff --git a/Source/core/svg/SVGGlyphRefElement.h b/Source/core/svg/SVGGlyphRefElement.h
index 62e1cfe8b7b255f277402e0a0816be830c439ac7..21a2e4ff5d389aa565f95239697cdf0e0c74feda 100644
--- a/Source/core/svg/SVGGlyphRefElement.h
+++ b/Source/core/svg/SVGGlyphRefElement.h
@@ -21,6 +21,7 @@
 #define SVGGlyphRefElement_h
 
 #if ENABLE(SVG_FONTS)
+#include "SVGNames.h"
 #include "core/svg/SVGElement.h"
 #include "core/svg/SVGURIReference.h"
 
@@ -64,6 +65,12 @@ private:
     float m_dy;
 };
 
+inline SVGGlyphRefElement* toSVGGlyphRefElement(Node* node)
+{
+    ASSERT_WITH_SECURITY_IMPLICATION(!node || node->hasTagName(SVGNames::glyphRefTag));
+    return static_cast<SVGGlyphRefElement*>(node);
+}
+
 }
 
 #endif