Fix channel mapping for 8-ch audio in ffmpeg_aac_bitstream_converter

FFmpeg's AVCodecContext->channels contains "raw number of channels"
(e.g. "2" for 2 channels, "8" for 8 channels). The change ensures that:
- the value of "8" is correctly mapped to 0b111 ADTS channel
config (aka MPEG-4 Channel Configuration),
- the value of "7" becomes unsupported.

BUG=640610

Review-Url: https://codereview.chromium.org/2273203003
Cr-Commit-Position: refs/heads/master@{#414685}
1 file changed