[selectors] Add :picture-in-picture pseudo-class #3796
Description
See https://wicg.github.io/picture-in-picture/#css-pseudo-class
The Picture-in-Picture WICG spec proposes adding a pseudo for the picture-in-picture element.
Blink has an intent-to-ship for this pseudo: https://groups.google.com/a/chromium.org/forum/#!msg/blink-dev/X-qPSmdSR_g/StUMNNiQAwAJ