According to the Meet Captain Phasma article by Vanity Fair yesterday it was Phasma chasing the Millenium Falcon in the trailer. So does that mean that this guy is actually Captain Phasma in a Tie pilot uniform?
My guess is they've got that part wrong. Just because it's chrome doesn't mean it's the same guy. For all we know, all the Tie Pilots are chrome, just as they were all black in the OT.
More likely than not, Phasma would have a fancy tie too if she has one at all, like the one that kills all those troopers (it has a different rectangular window to the rest).
I just think it would be kind of silly for the already armoured character to go through a costume change just to get in a Tie.
It's possible that they got it wrong, but it seems like quite the thing to get wrong, especially considering how much work is going into the marketing of this film, not to mention how careful they have been about revealing things.
The Vanity Fair article did however specifically say; "...that was the lanky Brienne of Tarth stalking through halls and chasing the Millennium Falcon in the most recent trailer."
Also I never assumed that the Tie pilot was wearing a chrome armour, it's just shiny looking during the few frames were the pilot fires at the Falcon. Also I don't see why she would wear the chrome armour if she actually went chasing after the Falcon in a Tie-fighter. It would make a lot more sense to put on a proper Tie pilot outfit. It might even be a necessity, considering the equipment in a Tie-fighter. For example that breathing gear seen in the trailer (as well as the OT) might be a necessity when piloting a Tie. Sure Vader didn't change outfit for his Tie piloting, but then again he already has a built in breathing gear.
But we'll just have to wait and see. I personally think that this Phasma character might turn out to be quite interesting, especially if she's chasing Han and Chewie throughout the film.