Test file produced with:
$ MP4Box -add "sample.mp4#video:colr=nclc,1,1,1" -new sample_18byte_nclx_colr.mp4

And then manually changing the `nclc` bytes to `nclx`.

This produces an 18-byte `colr` box with type `nclx`. The bitstream of
this file does not contain HDR content, so the file itself is invalid
for playback with a real decoder, but adding the box is enough to test
the extractor change in this commit.

(aside: MP4Box will let you pass `nclx`, but it requires 4 parameters, i.e. it
requires the full_range_flag to be set, resulting in a valid 19-byte colr box)

#minor-release
Issue: #9332
PiperOrigin-RevId: 405842520