Aah I've been seeing this posted everywhere lately.
The shot was filmed with a steadicam in front of the girl and flipped in post to mirror the image. The actual bathroom mirror had a bluescreen behind it (and was filmed at an angle) into which the original shot was keyed in.
This, you can tell especially if you notice the angle of her head and hair n the 2nd blue screen shot compared to the first where she is looking into the camera
They filmed the girl and the mirror separately. The mirror had a blue screen in the reflection, onto which they superimposed mirrored footage of the girl.
You can see the hard line where the mask goes to on the mirror's edge. someone posted it on their facebook a few days back, didn't take me too long but it still is some tough camera trickery to understand.