media: Support better decoder switching

media/filters/decoder_stream.h

Index: media/filters/decoder_stream.h
diff --git a/media/filters/decoder_stream.h b/media/filters/decoder_stream.h
index 97ab9705e342091e3e8cf6aa01b890da05e8a062..c9eed2203799f7f7e4b97488abd6de88aea5353e 100644
--- a/media/filters/decoder_stream.h
+++ b/media/filters/decoder_stream.h
@@ -50,14 +50,17 @@ class MEDIA_EXPORT DecoderStream {
     DECODE_ERROR,  // Decoder returned decode error.
   };
 
+  // Callback to create a list of decoders.
+  using CreateDecodersCB = base::RepeatingCallback<ScopedVector<Decoder>()>;
+
   // Indicates completion of a DecoderStream initialization.
-  typedef base::Callback<void(bool success)> InitCB;
+  using InitCB = base::Callback<void(bool success)>;
 
   // Indicates completion of a DecoderStream read.
-  typedef base::Callback<void(Status, const scoped_refptr<Output>&)> ReadCB;
+  using ReadCB = base::Callback<void(Status, const scoped_refptr<Output>&)>;
 
   DecoderStream(const scoped_refptr<base::SingleThreadTaskRunner>& task_runner,
-                ScopedVector<Decoder> decoders,
+                CreateDecodersCB create_decoders_cb,
                 MediaLog* media_log);
   virtual ~DecoderStream();
 
@@ -211,6 +214,10 @@ class MEDIA_EXPORT DecoderStream {
   // Whether |decoder_| has produced a frame yet. Reset on fallback.
   bool decoder_produced_a_frame_;
 
+  // Whether we are on a fallback decoder. Used to make sure we at most fallback

[watk, 2017/05/04 21:35:33]

What about instead of "on a fallback decoder" we say "using a fallback decoder"?
s/is_on_fallback_decoder_/using_fallback_decoder_/

s/we at most fallback once/we fall back once at most/

I think this is also a confusing name because after the fallback decoder
produces a frame we reset it to false. But it's still a fallback decoder? 

[xhwang, 2017/05/04 23:53:14]

How about has_fallen_back_once_on_decode_error_ ?

I choose this name to make sure we are talking about fallback on decode error,
not reinit error.

Also, I dropped the resetting code. Previously I thought we call fallback on
decode error again after reinit, but it's not the case (controlled by
decoder_produced_a_frame_). We can revisit this later. I have a TODO in the test
about this.

+  // once, to prevent issues like fallback infinitely.

[watk, 2017/05/04 21:35:33]

s/fallback infinitely/infinite fallback/

[xhwang, 2017/05/04 23:53:14]

Done.

+  bool is_on_fallback_decoder_;
+
   std::unique_ptr<DecryptingDemuxerStream> decrypting_demuxer_stream_;
 
   ConfigChangeObserverCB config_change_observer_cb_;