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 833b904a2459793b4ff86d5f85a1364ce7356feb..bf87b60e8da98a408548f24645bf5d20587da968 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