Introduce toSVGLinearGradientElement(), and use it

Source/core/svg/SVGLinearGradientElement.h

Index: Source/core/svg/SVGLinearGradientElement.h
diff --git a/Source/core/svg/SVGLinearGradientElement.h b/Source/core/svg/SVGLinearGradientElement.h
index 21bd87cb476ec4761ca776f1977205714606a042..c80d6ffeac59b04d0eb1e10959e3828a4021ab5f 100644
--- a/Source/core/svg/SVGLinearGradientElement.h
+++ b/Source/core/svg/SVGLinearGradientElement.h
@@ -21,6 +21,7 @@
 #ifndef SVGLinearGradientElement_h
 #define SVGLinearGradientElement_h
 
+#include "SVGNames.h"
 #include "core/svg/SVGAnimatedLength.h"
 #include "core/svg/SVGGradientElement.h"
 
@@ -53,6 +54,12 @@ private:
     END_DECLARE_ANIMATED_PROPERTIES
 };
 
+inline SVGLinearGradientElement* toSVGLinearGradientElement(Node* node)
+{
+    ASSERT_WITH_SECURITY_IMPLICATION(!node || node->hasTagName(SVGNames::linearGradientTag));
+    return static_cast<SVGLinearGradientElement*>(node);
+}
+
 } // namespace WebCore
 
 #endif