Add toRenderSVGResourceRadialGradient() fuction, and use it.

Source/core/rendering/svg/RenderSVGResourceRadialGradient.h

Index: Source/core/rendering/svg/RenderSVGResourceRadialGradient.h
diff --git a/Source/core/rendering/svg/RenderSVGResourceRadialGradient.h b/Source/core/rendering/svg/RenderSVGResourceRadialGradient.h
index f8a86e75aa7352a885a120cb32b7500eee8ec6eb..d00b8818d90a3fe38ae0afac04235f34b41d18b2 100644
--- a/Source/core/rendering/svg/RenderSVGResourceRadialGradient.h
+++ b/Source/core/rendering/svg/RenderSVGResourceRadialGradient.h
@@ -52,6 +52,12 @@ private:
     RadialGradientAttributes m_attributes;
 };
 
+inline RenderSVGResourceRadialGradient* toRenderSVGResourceRadialGradient(RenderSVGResourceContainer* resource)
+{
+    ASSERT_WITH_SECURITY_IMPLICATION(!resource || resource->resourceType() == RadialGradientResourceType);
+    return static_cast<RenderSVGResourceRadialGradient*>(resource);
+}
+
 }
 
 #endif