Create video and audio decoder threads on demand.

media/filters/ffmpeg_audio_decoder.h

Index: media/filters/ffmpeg_audio_decoder.h
diff --git a/media/filters/ffmpeg_audio_decoder.h b/media/filters/ffmpeg_audio_decoder.h
index edf93345bdec109c5f261c7e7b3ca2b235cd7004..eb4ea2d3bddaad8ef09dd6108c18292729aced05 100644
--- a/media/filters/ffmpeg_audio_decoder.h
+++ b/media/filters/ffmpeg_audio_decoder.h
@@ -15,10 +15,12 @@ struct AVCodecContext;
 namespace media {
 
 class DataBuffer;
+class MessageLoopFactory;
 
 class MEDIA_EXPORT FFmpegAudioDecoder : public AudioDecoder {
  public:
-  explicit FFmpegAudioDecoder(MessageLoop* message_loop);
+  FFmpegAudioDecoder(MessageLoopFactory* loop_factory,

[Ami GONE FROM CHROMIUM, 2012/03/08 16:32:07]

You could avoid the need for the MessageLoopFactory changes and the need for
MessageLoopFactoryForTest by instead passing a Callback<MessageLoop*()> to the
decoders, built with something like:
base::Bind(&MessageLoopFactory::GetMessageLoop, loop_factory,
"FooDecoderThread")
for the "real" callsite and 
template<class T> T Identity(T t) { return t; }
base::Bind(&Identity, MessageLoop::current())
for the test callsites.

[tommi (sloooow) - chröme, 2012/03/09 10:07:15]

That's so clever it can't possibly work! hehe, much better. Done!

+                     const std::string& loop_name);
   virtual ~FFmpegAudioDecoder();
 
   // AudioDecoder implementation.
@@ -55,6 +57,10 @@ class MEDIA_EXPORT FFmpegAudioDecoder : public AudioDecoder {
   // Delivers decoded samples to |read_cb_| and resets the callback.
   void DeliverSamples(const scoped_refptr<Buffer>& samples);
 
+  MessageLoop* EnsureMessageLoop();

[Ami GONE FROM CHROMIUM, 2012/03/08 16:32:07]

This idiom seems to me either racy or unnecessary.
If one is guaranteed that Initialize() is called before any other method, and
that that "before" carries memory-barrier implication, then every method other
than Initialize can simply trust & use message_loop_. 
If there is no such happens-before relationship then nothing prevents
EnsureMessageLoop() from running concurrently in different threads and returning
different values.

I believe the former is the case (and all methods other than Initialize() in the
.cc file can be reverted, and EML() removed).

[tommi (sloooow) - chröme, 2012/03/09 10:07:15]

Removed EML and now only initialize message_loop_ inside Initialize.
EML wasn't racy though since MessageLoopFactory is thread safe and always
returns the same pointer given the same key.

+
+  MessageLoopFactory* message_loop_factory_;
+  const std::string loop_name_;
   MessageLoop* message_loop_;
 
   scoped_refptr<DemuxerStream> demuxer_stream_;