[WebCodecs] Set ColorSpace properly for VP9 encoding.

This ensures that players/demuxers only looking at the bitstream
information at least have something meaningful. Really they should
be using the container information though.

R=eugene

Bug: 1377842
Change-Id: I64c8b78efdaa6f525bcf7b09ab8df94462d726ac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4029380


Reviewed-by: Dan Sanders <sandersd@chromium.org>
Commit-Queue: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1072409}