Add `x` and `y` to `DocumentPictureInPictureOptions`

[tomayac]

Since DocumentPictureInPicture.requestWindow() returns a Window which exposes moveTo(), I expected it to work, but in practice it doesn't. My use case is that I have a PiP dashboard that I like to move to a particular area of the screen, which of course is different from the default position in the lower right. Should this work? I was unsure whether to file a Chromium bug or a spec Issue, similar to #33. Sorry if it's more of an implementation problem.