[mediaqueries-5] Preference override interaction with iframes

[lukewarlow]

Need to work out the exact specifics with regards to iframes both same-origin and cross-origin.

Same-origin iframes should probably be updated with the parent frame's preference overrides but in an opaque manner.

e.g. if the parent frame sets colorScheme to dark then the iframe should see prefers-color-scheme as dark but shouldn't read navigator.preferences.colorScheme.value as dark.

Whereas, cross-origin iframes should maybe not be updated with the parent frame's preference overrides. This is an unfortunate limitation but is probably necessary to prevent new forms of data exfiltration.

[lukewarlow]

See for previous discussions: WICG/web-preferences-api#8