Re-add AC3/EAC3 audio demuxing support

media/media.gyp

Index: media/media.gyp
diff --git a/media/media.gyp b/media/media.gyp
index e4638021f83c748e4a78fc18f025bd271ea66aa7..0b9d7663b66ed693a64c92255934f6c6f06c6a4c 100644
--- a/media/media.gyp
+++ b/media/media.gyp
@@ -37,8 +37,12 @@
         # Enable HEVC/H265 demuxing. Actual decoding must be provided by the
         # platform.
         'enable_hevc_demuxing%': 1,
+        # Enable AC3/EAC3 audio demuxing. Actual decoding must be provided by the
+        # platform (or HDMI sink in Chromecast for audio pass-through case).
+        'enable_ac3_eac3_demuxing%': 1,
       }, {
         'enable_hevc_demuxing%': 0,
+        'enable_ac3_eac3_demuxing%': 0,
       }],
     ],
   },
@@ -1104,6 +1108,11 @@
             'formats/mp4/hevc.h',
           ],
         }],
+        ['proprietary_codecs==1 and enable_ac3_eac3_demuxing==1', {
+          'defines': [
+            'ENABLE_AC3_EAC3_DEMUXING'
+          ],
+        }],
         ['proprietary_codecs==1 and enable_hevc_demuxing==1 and media_use_ffmpeg==1', {
           'sources': [
             'filters/ffmpeg_h265_to_annex_b_bitstream_converter.cc',