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 81469aad6ef79c75a112f4cb426cf1bab6ff0f0a..b10dad5334f43229680676a4ab05bf5a0753ffd6 100644
--- a/media/media_options.gni
+++ b/media/media_options.gni
@@ -52,6 +52,11 @@ declare_args() {
   # Use low-memory buffers on non-Android builds of Chromecast.
   use_low_memory_buffer = is_chromecast && !is_android
 
+  # Enables AC3/EAC3 audio demuxing. This is enabled only on Chromecast, since
+  # it only provides demuxing, and is only useful for AC3/EAC3 audio
+  # pass-through to HDMI sink on Chromecast.
+  enable_ac3_eac3_demuxing = is_chromecast
+
   # Enables the MPEG2-TS stream parser for use with Media Source. Disabled by
   # default (except on Chromecast) since it's not available on the normal Web
   # Platform and costs money.