A fairly sizable portion of the Lab population does have a narrower chest. Some people fancy it over the thicker labs as a response to the extreme show Labs.
Google "Field bred Labrador" and you'll get quite a few narrower chested dogs, like these:

and

Labs have been so overbred that they can vary greatly from dog-dog.
If she were my dog, I'd just call her a Lab mix and call it good. As far as GSD or Rottie, I personally don't see characteristics of either popping out clearly and I'm not comfortable picking a mix without obvious traits showing.
I can say that I believe GSD would be more likely than Rottie due to the nature of her masking and tan points. Masking is dominant so one parent would've had to carried/expressed it and I don't think I've seen a Rottie with a mask let alone a strong one.(Maskless is recessive) While it's the opposite with GSDs, it's rare to find one without.
Edit: Here's a purebred Lab w/ Tan points for visual comparison
