Add toRenderSVGResourceFilter() fuction, and use it.

Source/core/rendering/svg/RenderSVGResourceFilter.h

Index: Source/core/rendering/svg/RenderSVGResourceFilter.h
diff --git a/Source/core/rendering/svg/RenderSVGResourceFilter.h b/Source/core/rendering/svg/RenderSVGResourceFilter.h
index 36b91ae9e92e89ed27c1a3615a560214a307f873..c03ccb82a68273d9b020b289f684f981177db78e 100644
--- a/Source/core/rendering/svg/RenderSVGResourceFilter.h
+++ b/Source/core/rendering/svg/RenderSVGResourceFilter.h
@@ -88,12 +88,18 @@ private:
     HashMap<RenderObject*, FilterData*> m_filter;
 };
 
-inline RenderSVGResourceFilter* toRenderSVGFilter(RenderObject* object)
+inline RenderSVGResourceFilter* toRenderSVGResourceFilter(RenderObject* object)
 {
     ASSERT_WITH_SECURITY_IMPLICATION(!object || object->isSVGResourceFilter());
     return static_cast<RenderSVGResourceFilter*>(object);
 }
 
+inline RenderSVGResourceFilter* toRenderSVGResourceFilter(RenderSVGResourceContainer* resource)
+{
+    ASSERT_WITH_SECURITY_IMPLICATION(!resource || resource->resourceType() == FilterResourceType);
+    return static_cast<RenderSVGResourceFilter*>(resource);
+}
+
 }
 
 #endif