Add toSVGStyleElement(), and use it.

Source/core/svg/SVGStyleElement.h

Index: Source/core/svg/SVGStyleElement.h
diff --git a/Source/core/svg/SVGStyleElement.h b/Source/core/svg/SVGStyleElement.h
index e263f90b0c410e96a836c27b455e6c0f900f5302..35cf7be6a1e0c621d9c22cb64e9875a75676bced 100644
--- a/Source/core/svg/SVGStyleElement.h
+++ b/Source/core/svg/SVGStyleElement.h
@@ -21,6 +21,7 @@
 #ifndef SVGStyleElement_h
 #define SVGStyleElement_h
 
+#include "SVGNames.h"
 #include "core/dom/StyleElement.h"
 #include "core/svg/SVGElement.h"
 
@@ -67,6 +68,12 @@ private:
     Timer<SVGElement> m_svgLoadEventTimer;
 };
 
+inline SVGStyleElement* toSVGStyleElement(Node* node)
+{
+    ASSERT_WITH_SECURITY_IMPLICATION(!node || node->hasTagName(SVGNames::styleTag));
+    return static_cast<SVGStyleElement*>(node);
+}
+
 } // namespace WebCore
 
 #endif // SVGStyleElement_h