Re-add AC3/EAC3 audio demuxing support

build/common.gypi

Index: build/common.gypi
diff --git a/build/common.gypi b/build/common.gypi
index abfa247f63c1c9da79cb492be741815cf0799582..35ba18db93402a767b7b5f69c381971838d08ac4 100644
--- a/build/common.gypi
+++ b/build/common.gypi
@@ -1468,6 +1468,9 @@
     # Force disable libstdc++ debug mode.
     'disable_glibcxx_debug%': 0,
 
+    # Set to 1 to enable AC3/EAC3 audio demuxing

[DaleCurtis, 2015/01/14 23:36:33]

Do we need this here? I don't see any non media/ code using this.

[servolk, 2015/01/14 23:44:11]

Two reasons:
1. I was trying to keep gyp variables consistent, enable_ac3_eac3_audio_demuxing
is very similar in its role and scope to enable_mpeg2ts_stream_parser and that
one is defined here.
2. We will actually needs this in net/base/mime_types.cc when adding ac3/eac3 to
the list of supported mime types.
This CL is not complete yet - I still need to finish the net/base/mime_types.cc
part and trying to figure out if I can add more unit tests (the current version
only has unit tests for FFmpeg demuxer, would be nice to add some unit tests for
MSE code path). But I've decided to sent it out a bit earlier to give you guys
time to take a look at this, I didn't expect such a quick reply :)

[DaleCurtis, 2015/01/14 23:50:07]

New values of this type are effectively banned, so you'll face an uphill battle
trying to add this. Each define added here is applied to every Chrome build
file. You're also missing the GN changes, both of which will require brettw@'s
approval (who's very likely going to say what I just said above).

In this case it's only for Chromecast, so you might be able to get away with it,
but I'd look closely at how you can avoid this being a global value.

I believe for the net/ stuff we want to move that into media/ (certainly the
net/ team wants us to).

[servolk, 2015/01/15 00:06:45]

Added the same option to media/media_options.gni for now. But I don't see where
the mpeg2ts option declared in the .gni file is being used.
What would be a good alternative way to achieve this (ac3/eac3 demuxing behind a
gyp/gn variable, so that we could only enable it when necessary, e.g.
Chromecast)?
 
Yes, I understand and completely agree here, it seemed strange to me that net/
has such a hard dependency on media, I agree that this stuff has to move to
media. But at the moment the first level of mime type checks is still done
there, we have to add AC3/EAC3 mime types in there to make them usable, and then
it would be totally fine to move all this code to media/ in a separate CL,
together with all the other media mime types.

[DaleCurtis, 2015/01/15 01:51:01]

If it's only necessary in media/ and net/ I'd just duplicate the logic in both
net/ and media/ .gyp/.gn files temporarily until everything can be moved into
media/. That said, I bet net/ folk will say we should just move the code first,
they've been asking forever.

[servolk, 2015/01/15 02:37:20]

Ok, I can try to move the media mime checks from net/ to media/, but could you
give me a bit more context?
I guess I could always just try a brute-force approach, e.g. just extract all
media-related code into a separate class, name it MediaMimeUtil, move it
somewhere under media/ and use MediaMimeUtil in the net/'s
MimeUtil::IsSupportedMediaMimeType and MimeUtil::IsSupportedNonImageMimeType.
But is that what you and net/ folks had in mind?
If there has been a discussion on that matter already, could you tell me the bug
# or loop me in on that mail thread.

[DaleCurtis, 2015/01/15 17:56:13]

+rsleevi,wolenetz who are more familiar with the historical context here. I
think it's mostly a straightforward move, but it may be necessary to move it
into a minimal media/ subtarget that net/ could depend upon if any functionality
is still necessary there.  I thought there was a bug for this, but I can't find
one now. 

+    'enable_ac3_eac3_audio_demuxing%': 0,
+
     # Set to 1 to compile with MSE support for MPEG2 TS
     'enable_mpeg2ts_stream_parser%': 0,
 
@@ -1805,6 +1808,7 @@
         'use_system_fontconfig%': 1,
       }],
       ['chromecast==1', {
+        'enable_ac3_eac3_audio_demuxing%': 1,
         'enable_mpeg2ts_stream_parser%': 1,
         'ffmpeg_branding%': 'ChromeOS',
         'ozone_platform_ozonex%': 1,
@@ -2647,6 +2651,9 @@
           ['enable_mpeg2ts_stream_parser==1', {
             'defines': ['ENABLE_MPEG2TS_STREAM_PARSER'],
           }],
+          ['enable_ac3_eac3_audio_demuxing==1', {
+            'defines': ['ENABLE_AC3_EAC3_AUDIO_DEMUXING'],
+          }],
         ],
       }],
       ['enable_viewport==1', {