The libaom av1 decoder can return high bit depth frame data now. Handle those frames by downsampling them to 8 bits per channel so they can be passed to our normal playback pipeline.
When av1 video playback is enabled, declare it as supported in the webm container in MediaSource.IsTypeSupported.
Also support special mime types of the form video/webm; codecs=vp9.experimental.<git-commit-id> so test sites can verify playback support of particular encodings while the av1 bitstream is under development.