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

Source/core/svg/SVGImageElement.h

Index: Source/core/svg/SVGImageElement.h
diff --git a/Source/core/svg/SVGImageElement.h b/Source/core/svg/SVGImageElement.h
index 9f78f4068e906afa30d08df77642a181bbedc6fb..8425703e01df060979ab8a5638afe65e62a4beae 100644
--- a/Source/core/svg/SVGImageElement.h
+++ b/Source/core/svg/SVGImageElement.h
@@ -76,9 +76,14 @@ private:
     SVGImageLoader m_imageLoader;
 };
 
+inline bool isSVGImageElement(const Node* node)
+{
+    return node->hasTagName(SVGNames::imageTag);
+}
+
 inline SVGImageElement* toSVGImageElement(Node* node)
 {
-    ASSERT_WITH_SECURITY_IMPLICATION(!node || node->hasTagName(SVGNames::imageTag));
+    ASSERT_WITH_SECURITY_IMPLICATION(!node || isSVGImageElement(node));
     return static_cast<SVGImageElement*>(node);
 }