That was Nico's mother, Tina
It's been reported for ages that Linda Louise Duan was cast as Tina Minoru in the film; Duan's personal website reported in late October 2016 on a promo piece in the Essex County Standard:
MoviePilot.com also had an interview with Duan in October 2016, where they claim that her scenes were cut from the final film; evidently she initially had a larger role then what we ended up seeing. The article includes the following clip of her:
Nico and Tina are both confirmed for the Runaways TV show
Marvel released some of their casting decisions early in February 2017, and both Nico and Tina have been cast:
Nico will be portrayed by Lyrica Okana:
Lyrica Okano ("The Affair," "Unforgettable") Nico Minoru – tough, intelligent, and independent–embodies teenage angst. A budding “Wiccan,” Nico’s carefully crafted goth appearance isolates her from her peers and family, but maybe what she really needs is someone to talk to.
Tina will be portrayed by Brittany Ishibashi:
Brittany Ishibashi ("This Is Us," "Teenage Mutant Ninja Turtles: Out of the Shadows") as Tina Minoru – a perfectionist "tiger mom." In her professional life, she is a brilliant innovator and ruthless CEO. Emotionally, she is less confident, often shying away from difficult conversations with her daughter Nico and careful to avoid explicit displays of her feelings.
How Tina goes from Doctor Strange to The Runaways has yet to be revealed.
Why is Tina allied with the "Good Guys"?
In the interview I linked to above, Duan states that the Tina Minoru we see in Doctor Strange is a younger version of the character:
The Tina in Doctor Strange is obviously a younger Tina than in The Runaways, so that gave me space for my own interpretation of who she is. She has never been explored before at this point in her life, not even in The Runaways comics.
This is somewhat borne out by the casting announcement I linked above; while Duan is a fairly young actress (her exact age appears not to be public, but her IMDB biography states that she graduated acting school in 2015), while Ishibashi is rather older (born in 1980)
Presumably, as with Baron Mordo, the Tina seen in the film has yet to experience her "Start of Darkness"1.
1 Warning, TVTropes link. It's dangerous to go alone