Because we used decrementing negative source ids for fake video id when
instantating a native macOS screen share picker, we eventually hit the
`DesktopMediaID::kFakeId = -3` in Chromium source code which displayed a
test green screen.
In this change we reserve our own fake id of `-4` and decrement the
window id integer for uniqueness instead.
Co-authored-by: Fedor Indutny <238531+indutny@users.noreply.github.com>
6aa6bada79