Years of fear has conditioned cats into assuming the worst when it comes to the intentions of a dog. They try to make up for it in charm what they lack in strength and it can work sometimes. In this case the Rottweiler has allowed the cat safe sanctuary against the elements. This content cat has all its paws up in the air, it has completely dropped all its guards because it has placed complete trust into the dog.
The dog knows full well that the cat poses no harm, it is well fed, safe from the threat of predators around it and the cat actually turned out to be a good ally. This make sit incredibly easy for them both to be consider each other as the best of friends instead of natural enemies.