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 cedcdf7132793b16af8af32d28b0a981f2d924e6..5191517c9ed2d5284411933391d65248b6e531dd 100644
--- a/media/media_options.gni
+++ b/media/media_options.gni
@@ -44,10 +44,18 @@ declare_args() {
   # platform. Enable by default for Chromecast.
   enable_hevc_demuxing = proprietary_codecs && is_chromecast
 
+  # Enable HLS with SAMPLE-AES decryption. Actual decryption must be provided by
+  # the platform. Enabled by default on the cast desktop implementation to allow
+  # unit tests of the MP2TS parsing support.
+  enable_hls_sample_aes = proprietary_codecs && is_cast_desktop_build
+
   # TODO(GYP): This should be a platform define.
   is_openbsd = false
 }
 
+# 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.