Add toRenderSVGResourceFilter() fuction, and use it.

Source/core/rendering/svg/SVGRenderTreeAsText.cpp

Index: Source/core/rendering/svg/SVGRenderTreeAsText.cpp
diff --git a/Source/core/rendering/svg/SVGRenderTreeAsText.cpp b/Source/core/rendering/svg/SVGRenderTreeAsText.cpp
index 8a1d99d38e250c23362a792bfd6096a63c33d83e..ee964e5af22ea3d5ad45d6017f5d2b0bfe32754e 100644
--- a/Source/core/rendering/svg/SVGRenderTreeAsText.cpp
+++ b/Source/core/rendering/svg/SVGRenderTreeAsText.cpp
@@ -492,7 +492,7 @@ void writeSVGResourceContainer(TextStream& ts, const RenderObject& object, int i
         writeNameValuePair(ts, "maskContentUnits", masker->maskContentUnits());
         ts << "\n";
     } else if (resource->resourceType() == FilterResourceType) {
-        RenderSVGResourceFilter* filter = static_cast<RenderSVGResourceFilter*>(resource);
+        RenderSVGResourceFilter* filter = toRenderSVGResourceFilter(resource);
         writeNameValuePair(ts, "filterUnits", filter->filterUnits());
         writeNameValuePair(ts, "primitiveUnits", filter->primitiveUnits());
         ts << "\n";