Ingredients injected into chicken breast
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.

However

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?

Sources:


http://notinmyfood.org/posts/2839-what-are-they-pumping-into-your-chicken
http://www.pawnation.com/2013/08/16/what-can-dogs-drink/4
http://www.love61.com/other/pet-0347.html 
https://www.vetlearn.com/_preview?_cms.fe.previewId=88ca0770-ed59-11e2-8f1a-005056ad4734&amp;ArticleURL=https://www.vetlearn.com/compendium/hidden-dangers-in-the-kitchen-common-foods-toxic-to-dogs-and-cats