Add toRenderSVGResourceMarker() fuction, and use it.

Source/core/rendering/svg/RenderSVGResourceMarker.h

Index: Source/core/rendering/svg/RenderSVGResourceMarker.h
diff --git a/Source/core/rendering/svg/RenderSVGResourceMarker.h b/Source/core/rendering/svg/RenderSVGResourceMarker.h
index 528f1f1d2ba65ba84ca0640923b242ea136f3e93..560e654ee50eb021cd728b8c26db2c78d3a02b0c 100644
--- a/Source/core/rendering/svg/RenderSVGResourceMarker.h
+++ b/Source/core/rendering/svg/RenderSVGResourceMarker.h
@@ -75,6 +75,12 @@ private:
     FloatRect m_viewport;
 };
 
+inline RenderSVGResourceMarker* toRenderSVGResourceMarker(RenderSVGResource* resource)
+{
+    ASSERT_WITH_SECURITY_IMPLICATION(!resource || resource->resourceType() == MarkerResourceType);
+    return static_cast<RenderSVGResourceMarker*>(resource);
+}
+
 }
 
 #endif