Add toRenderSVGResourceMarker() 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..61e26c3218cd73faa3982de564f8dd2dd6503732 100644
--- a/Source/core/rendering/svg/SVGRenderTreeAsText.cpp
+++ b/Source/core/rendering/svg/SVGRenderTreeAsText.cpp
@@ -511,7 +511,7 @@ void writeSVGResourceContainer(TextStream& ts, const RenderObject& object, int i
         writeNameValuePair(ts, "clipPathUnits", clipper->clipPathUnits());
         ts << "\n";
     } else if (resource->resourceType() == MarkerResourceType) {
-        RenderSVGResourceMarker* marker = static_cast<RenderSVGResourceMarker*>(resource);
+        RenderSVGResourceMarker* marker = toRenderSVGResourceMarker(resource);
         writeNameValuePair(ts, "markerUnits", marker->markerUnits());
         ts << " [ref at " << marker->referencePoint() << "]";
         ts << " [angle=";