Add toRenderSVGResourceClipper() 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 f00c1383f51710496f13e8bbce42cd36dde11282..37b8eff9561aac7ba6089716a1b3d9d4c998ab8b 100644
--- a/Source/core/rendering/svg/SVGRenderTreeAsText.cpp
+++ b/Source/core/rendering/svg/SVGRenderTreeAsText.cpp
@@ -507,8 +507,7 @@ void writeSVGResourceContainer(TextStream& ts, const RenderObject& object, int i
                 lastEffect->externalRepresentation(ts, indent + 1);
         }
     } else if (resource->resourceType() == ClipperResourceType) {
-        RenderSVGResourceClipper* clipper = static_cast<RenderSVGResourceClipper*>(resource);
-        writeNameValuePair(ts, "clipPathUnits", clipper->clipPathUnits());
+        writeNameValuePair(ts, "clipPathUnits", toRenderSVGResourceClipper(resource)->clipPathUnits());
         ts << "\n";
     } else if (resource->resourceType() == MarkerResourceType) {
         RenderSVGResourceMarker* marker = static_cast<RenderSVGResourceMarker*>(resource);