MP4 support for Common Encryption 'cbcs' scheme.

In order to support SAMPLE-AES encrypted packed audio streams
in HLS, the Cast media player library cast will reformat
them into MP4. The change herein, by adding support for
the 'cbcs' scheme will allow them to be decrypted by the platform.

BUG=568326

Review-Url: https://codereview.chromium.org/1998333002
Cr-Commit-Position: refs/heads/master@{#435739}
9 files changed