MP4 support for Common Encryption 'cbcs' scheme.

media/media_options.gni

Index: media/media_options.gni
diff --git a/media/media_options.gni b/media/media_options.gni
index 61dac760ac3aad5cda26b90799147d60ad743515..a7ab2145d0df6e8cf5510fc96ee5d604599d02a2 100644
--- a/media/media_options.gni
+++ b/media/media_options.gni
@@ -41,6 +41,10 @@ declare_args() {
   enable_mse_mpeg2ts_stream_parser =
       (proprietary_codecs && is_chromecast) || use_libfuzzer || use_afl
 
+  # Enable support for the 'cbcs' encryption scheme added by MPEG Common
+  # Encryption 3rd Edition (ISO/IEC 23001-7), published 02/15/2016.
+  enable_cbcs_encryption_scheme = is_chromecast
+
   # Enable HEVC/H265 demuxing. Actual decoding must be provided by the
   # platform. Enable by default for Chromecast.
   enable_hevc_demuxing = proprietary_codecs && is_chromecast