It's hard to say. It's not like the officer was just strolling past and the dog suddenly charged him out of the blue. He was walking back and forth in the middle of the street, effectively taunting the dog, which in all probability was just protecting his home and his masters from what he perceived was a danger. I work a lot with large dogs, and there are many, many good dogs out there that would have done the same thing in that situation.
Of course, that does NOT mean it's OK for the dog to attack the cop. Legally, the owner is still responsible for restraining the dog to ensure everybody's safety. I think everybody here was pretty much taken by surprise by what happened, and weren't expecting it. If the cop had been just walking along like any other pedestrian, nothing would have happened.