Fix VideoDecoderShim to return correct decode_id for all codecs.

media/base/video_decoder.h

Index: media/base/video_decoder.h
diff --git a/media/base/video_decoder.h b/media/base/video_decoder.h
index d7e7a791de3d168668476cbdbd73aae22dd90ef4..b8b8fa71f71484ba84552d17ea0cf474f0460fe1 100644
--- a/media/base/video_decoder.h
+++ b/media/base/video_decoder.h
@@ -78,8 +78,9 @@ class MEDIA_EXPORT VideoDecoder {
   // called again).
   //
   // After decoding is finished the decoder calls |output_cb| specified in
-  // Initialize() for each decoded frame. |output_cb| may be called before or
-  // after |decode_cb|.
+  // Initialize() for each decoded frame. In general |output_cb| may be called
+  // before or after |decode_cb|, but software decoders must always call
+  // |output_cb| before calling |decode_cb|.

[xhwang, 2015/01/15 21:35:10]

Hmm, is there any reason why VideoDecoderShim can't work with the current
VideoDecoder interface and has to have multiple exceptions?

[Sergey Ulanov, 2015/01/15 22:59:20]

PPB_VideoDecoder API needs to tell the plugin which frame is generated by which
input buffer. VideoDecoder interface currently doesn't support that.
VideoDecoder just calls OutputCB, but it's not possible to tell which buffer the
frame comes from. Another possible solution is to add decode_id parameter in
Decode() and OutputCB, but that would also require updating AudioDecoder
(because DecoderStream is shared), and would touch a lot of code, while this
feature is needed only in VideoDecoderShim

[xhwang, 2015/01/15 23:41:47]

Thanks for the info! But what exactly do you mean here by "software decoders
must always call |output_cb| before calling |decode_cb|"? What if the software
has decoding delay? e.g.

input 1, no output, decode_cb called
input 2, no output, decode_cb called
input 3, output 1, decode_cb called

In the last line, |output_cb| is called before |decode_cb|, is it okay? But the
|decode_cb| is also called before the future |output_cb| for input 3. So this is
kinda ambiguous...

   //
   // If |buffer| is an EOS buffer then the decoder must be flushed, i.e.
   // |output_cb| must be called for each frame pending in the queue and