Re-add AC3/EAC3 audio demuxing support

media/media_options.gni

Index: media/media_options.gni
diff --git a/media/media_options.gni b/media/media_options.gni
index c2b470930784a99c50d4efe48b7f642365d3c39f..ca1371d1cd147b40d15651b1c36626f6fa4716a5 100644
--- a/media/media_options.gni
+++ b/media/media_options.gni
@@ -41,6 +41,11 @@ declare_args() {
     }
   }
 
+  # Enables AC3/EAC3 audio demuxing. Disabled by default because it's only
+  # demuxing, not decoding. It's of limited use (but can be used to implement
+  # AC3/EAC3 audio pass-through)
+  enable_ac3_eac3_audio_demuxing = false
+
   # Enables the MPEG2-TS stream parser for use with Media Source. Disabled by
   # default since it's not available on the normal Web Platform and costs money.
   enable_mpeg2ts_stream_parser = false