Introduce toSVGGradientElement(), use it

Source/core/svg/SVGGradientElement.h

Index: Source/core/svg/SVGGradientElement.h
diff --git a/Source/core/svg/SVGGradientElement.h b/Source/core/svg/SVGGradientElement.h
index 09cb256799e3a245530b6610c0806126d433a1b9..dc578af8df0b80dce5f0c134848e1d873c2f9e77 100644
--- a/Source/core/svg/SVGGradientElement.h
+++ b/Source/core/svg/SVGGradientElement.h
@@ -21,6 +21,7 @@
 #ifndef SVGGradientElement_h
 #define SVGGradientElement_h
 
+#include "SVGNames.h"
 #include "core/platform/graphics/Gradient.h"
 #include "core/svg/SVGAnimatedBoolean.h"
 #include "core/svg/SVGAnimatedEnumeration.h"
@@ -106,6 +107,12 @@ private:
     END_DECLARE_ANIMATED_PROPERTIES
 };
 
+inline SVGGradientElement* toSVGGradientElement(Node* node)
+{
+    ASSERT_WITH_SECURITY_IMPLICATION(!node || (node->hasTagName(SVGNames::radialGradientTag) || node->hasTagName(SVGNames::linearGradientTag)));
+    return static_cast<SVGGradientElement*>(node);
+}
+
 } // namespace WebCore
 
 #endif