Add toRenderSVGResourceClipper() fuction, and use it.

Source/core/rendering/svg/RenderSVGResourceClipper.h

Index: Source/core/rendering/svg/RenderSVGResourceClipper.h
diff --git a/Source/core/rendering/svg/RenderSVGResourceClipper.h b/Source/core/rendering/svg/RenderSVGResourceClipper.h
index 7344e7eff60f9643a574e1de3a28fdd844878fe4..a5be4ff6828093c8c0d15c097479482f4fd11750 100644
--- a/Source/core/rendering/svg/RenderSVGResourceClipper.h
+++ b/Source/core/rendering/svg/RenderSVGResourceClipper.h
@@ -73,6 +73,12 @@ private:
     bool m_inClipExpansion;
 };
 
+inline RenderSVGResourceClipper* toRenderSVGResourceClipper(RenderSVGResource* resource)
+{
+    ASSERT_WITH_SECURITY_IMPLICATION(!resource || resource->resourceType() == ClipperResourceType);
+    return static_cast<RenderSVGResourceClipper*>(resource);
+}
+
 }
 
 #endif