Add toSVGFEMergeNodeElement() fuction, and use it.

Source/core/svg/SVGFEMergeNodeElement.h

Index: Source/core/svg/SVGFEMergeNodeElement.h
diff --git a/Source/core/svg/SVGFEMergeNodeElement.h b/Source/core/svg/SVGFEMergeNodeElement.h
index bd90db7c60fd9bdc8dddf3968dcc8dc841464367..9bfe59aec6ee297a1f3105ec527116b6db542732 100644
--- a/Source/core/svg/SVGFEMergeNodeElement.h
+++ b/Source/core/svg/SVGFEMergeNodeElement.h
@@ -21,6 +21,7 @@
 #ifndef SVGFEMergeNodeElement_h
 #define SVGFEMergeNodeElement_h
 
+#include "SVGNames.h"
 #include "core/svg/SVGAnimatedString.h"
 #include "core/svg/SVGElement.h"
 
@@ -44,6 +45,12 @@ private:
     END_DECLARE_ANIMATED_PROPERTIES
 };
 
+inline SVGFEMergeNodeElement* toSVGFEMergeNodeElement(Node* node)
+{
+    ASSERT_WITH_SECURITY_IMPLICATION(!node || node->hasTagName(SVGNames::feMergeNodeTag));
+    return static_cast<SVGFEMergeNodeElement*>(node);
+}
+
 } // namespace WebCore
 
 #endif