After a good search of the internet I could not actually find the ingredients of the broth. I think, as its a broth your dog will probably be perfectly fine.
However I've also been reading another post that seems to be totally against giving broth.. So I took a look for the toxic level for Dog's and found this:
The toxic dose has been found to be as low as 5 g/kg in cats and 15 g/kg in dogs.2 A medium-sized onion (2 to 3¼” in diameter) is approximately 150 g, and the average weight of a garlic clove is 6 g. A 10-lb dog would ingest a potentially toxic dose by eating only half of an onion.
So a Dog eating for example a 200g chicken breast pumped with 30% weight chicken stock (60g Chicken Stock) I'd hazard a guess at around 2g of the stock is Onion residue. So probably perfectly safe.
I would be more worried/concerned about the sodium content of these 'enhanced' chicken breasts. I've read on one page that the sodium in some chicken is 20% (440 mg) of an adult human's daily intake which I'm no vet but I'm guessing (I can't seem to find a figure anywhere) is close if not way more than a dog should be having.
Might be worth just buying premium dog food if you fancy treating your pooch?