Support HLS MPEG2 TS with SAMPLE-AES encryption.

media/media_options.gni

Index: media/media_options.gni
diff --git a/media/media_options.gni b/media/media_options.gni
index 21d31d0ee947dbf6b8b5d7da1019b1fd8acb5be8..8d9126f48f7ef905812874dc9675eafc46e84e71 100644
--- a/media/media_options.gni
+++ b/media/media_options.gni
@@ -42,8 +42,16 @@ declare_args() {
   # Enable HEVC/H265 demuxing. Actual decoding must be provided by the
   # platform. Enable by default for Chromecast.
   enable_hevc_demuxing = proprietary_codecs && is_chromecast
+
+  # Enable HLS with SAMPLE-AES decryption.
+  # Enabled by default on the cast desktop implementation to allow unit tests of
+  # MP2TS parsing support.
+  enable_hls_sample_aes = proprietary_codecs && is_cast_desktop_build
 }
 
+# enable_hls_sample_aes can only be true if enable_mse_mpeg2ts_stream_parser is.
+assert(enable_mse_mpeg2ts_stream_parser || !enable_hls_sample_aes)
+
 # Use a second declare_args() to pick up possible overrides of |use_cras|.
 declare_args() {
   # Enables runtime selection of PulseAudio library.