Support (E)AC3 passthrough

media/base/audio_codecs.h

Index: media/base/audio_codecs.h
diff --git a/media/base/audio_codecs.h b/media/base/audio_codecs.h
index 1219d7acf24e5f3aab09f63860229d5e7c94692d..db4e0127261289397d3c69f70bbc39a499fe6fe5 100644
--- a/media/base/audio_codecs.h
+++ b/media/base/audio_codecs.h
@@ -32,13 +32,14 @@ enum AudioCodec {
   kCodecPCM_ALAW = 14,
   kCodecALAC = 15,
   kCodecAC3 = 16,
+  kCodecRaw = 17,

[DaleCurtis, 2016/11/01 23:05:13]

I don't think this is quite what we want. The correct codec_id should be used,
but instead of going through the normal decoder we should have a
PassthroughAudioDecoder inserted in the decoder list which returns AudioBuffers
with a SampleFormat::MP3, SampleFormat::AAC, SampleFormat::EAC3, etc.

For unencrypted buffers we should be able to pipe the demuxer output straight
into an AudioBuffer. For encrypted buffers, there's no reason the
MediaCodecAudioDecoder can't return buffers of that type.

[AndyWu, 2016/11/04 18:04:24]

Let's focus on encrypted case first, since that's what we have in HBOGo. I agree
with you that we can optimize unencrypted case.

The major difficulty is that we have to make sure raw audio decoder and raw
audio output stream are enabled at the same time. Right now, it is decided on
MediaClient::IsPassthroughAudioSupported(), which is not accessible in
MediaCodecAudioDecoder.

Base on your comments, what I can thing of is creating
MojoPassthroughAudioDecoder, which is a wrapper of MojoAudioDecoder. It would
use MediaClient::IsPassthroughAudioSupported() to determine whether to create
MojoAudioDecoder or just return unsupported. We also need to add one field in
AudioDecoderConfig to notify MediaCodecAudioDecoder to use raw decoder or not.

Does it sounds good to you? Any suggestion?

[DaleCurtis, 2016/11/04 21:17:33]

I think whether Raw is used or not needs to be part of the AudioDecoderConfig,
not a codec name. This will allow us to flip in and out of raw mode with a
decoder restart if necessary (for future stuff) and avoids polluting the codec
enums.

Plumbing that change can be part of a separate CL too for easier review. Then
you'll get the flag in all the right places.

[AndyWu, 2016/11/08 00:04:21]

OK, done. I will create a separate CL for review.

   // DO NOT ADD RANDOM AUDIO CODECS!
   //
   // The only acceptable time to add a new codec is if there is production code
   // that uses said codec in the same CL.
 
   // Must always be equal to the largest entry ever logged.
-  kAudioCodecMax = kCodecAC3,
+  kAudioCodecMax = kCodecRaw,
 };
 
 std::string MEDIA_EXPORT GetCodecName(AudioCodec codec);