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 7307730573bdfc8db9d7312d2a3683a4898515a4..3aab0a04970f66f4e334c9beb5929ddb5b38e84d 100644
--- a/media/media_options.gni
+++ b/media/media_options.gni
@@ -50,8 +50,16 @@ declare_args() {
   enable_hevc_demuxing = proprietary_codecs && is_chromecast
 
   enable_webrtc = !is_ios
+
+  # 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.