message list [nfc]: Make the BackgroundData type exact.
message list [nfc]: Make the BackgroundData type exact.

I don't think there was ever a reason we'd have wanted this to be
inexact.  It would have been missed by our various automated sweeps
to make object types exact, though, because its name isn't in a
common pattern like `Props` or `State`.

vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile