make safari real devices work with new commandProxy model (fix #3376)

cc @sebv
@snevesbarros this is what I think is going on, please take a look and tell me whether you agree
@Gersee can you also pull this into your local copy and see if it helps?
@jlipps I applied your fix locally and its still freezing. I think it's because instruments.start never actually makes the call back, so this cb() is never hit.