Introduce isSVGFontElement() and isSVGImageElement(), and use them

Source/core/svg/SVGFontElement.h

Index: Source/core/svg/SVGFontElement.h
diff --git a/Source/core/svg/SVGFontElement.h b/Source/core/svg/SVGFontElement.h
index ff9ae3a2e60c5a6e5fcf6e15f7f299181ce61056..8bcf28960c347be5bbabdc4a19f094fa73aa2551 100644
--- a/Source/core/svg/SVGFontElement.h
+++ b/Source/core/svg/SVGFontElement.h
@@ -89,9 +89,14 @@ private:
     bool m_isGlyphCacheValid;
 };
 
+inline bool isSVGFontElement(const Node* node)
+{
+    return node->hasTagName(SVGNames::fontTag);
+}
+
 inline SVGFontElement* toSVGFontElement(Node* node)
 {
-    ASSERT_WITH_SECURITY_IMPLICATION(!node || node->hasTagName(SVGNames::fontTag));
+    ASSERT_WITH_SECURITY_IMPLICATION(!node || isSVGFontElement(node));
     return static_cast<SVGFontElement*>(node);
 }