Support BT470BG untyped matrix from macOS.

For matrices that macOS doesn't recognize it'll return a value of the
form "YCbCrMatrix#n" where n seems to correspond to the ISO
23001-8:2016 id; see VideoColorSpace::MatrixID. We could add
generalized code to handle this, but it's rarely seen so favor
simplicity.

This mostly just documents how macOS handles unknown matrices in
case its relevant to a future reader.

R=ccameron

Change-Id: Ibd78200888c77cb1cd6d51d12e9f45e028322789
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6495005
Reviewed-by: ccameron chromium <ccameron@chromium.org>
Commit-Queue: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1454058}