Restore bit depth check in LibvpxVp9Decoder

ReturnFrame() used to reject decoded images with bit depths other than 8
and 10. The check was lost when it started to switch on img->fmt instead
of img->bit_depth in
https://webrtc-review.googlesource.com/c/src/+/266180. Since then,
12-bit frames (VP9 profile 2 and 3) are wrapped in 10-bit buffers and,
for BT.2020 content, hit RTC_DCHECK_NOTREACHED() in
ExtractVP9ColorSpace().

Return WEBRTC_VIDEO_CODEC_NO_OUTPUT for such frames again, since no
frame buffer type can represent 12-bit content.

Fixed: chromium:569701823
Change-Id: I7ba5437dfeffc581c2dbc516ab492252928cdc67
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/507600
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48799}