Once the Cast dialog supports showing disabled sinks, we'd want to show mirrored displays as disabled on the sinks list, and show a tooltip mentioning that the display needs to exit mirroring in order to be presented to.

We'll need to look into how each OS handles mirrored displays, e.g.:
https://cs.chromium.org/chromium/src/ui/display/mac/screen_mac.mm?sq=package:chromium&l=301