Allow multiple concurrent Decode() requests in VideoDecoder interface.

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 5b6664af4fb0b40a37bf6dfd2c498eff77b1ba97..93a82e164310005f35a5af4050d5290a14ce2dd9 100644
--- a/media/base/video_decoder.h
+++ b/media/base/video_decoder.h
@@ -46,8 +46,10 @@ class MEDIA_EXPORT VideoDecoder {
                           const PipelineStatusCB& status_cb) = 0;
 
   // Requests a |buffer| to be decoded. The status of the decoder and decoded
-  // frame are returned via the provided callback. Only one decode may be in
-  // flight at any given time.
+  // frame are returned via the provided callback. Some decoders may allow
+  // decoding multiple frames in parallel. Callers should call
+  // GetMaxDecodeRequests() to get number of frames that may be decoded in
+  // parallel.

[Ami GONE FROM CHROMIUM, 2014/04/16 01:00:00]

An alternative approach that may work better is to return true/false depending
on whether the decoder was able to accept the request.
(better for the client who doesn't have to track the # of outstanding requests?)

[Sergey Ulanov, 2014/04/16 01:44:14]

Isn't DecodeStream the only client of this interface?
Also I think GetMaxDecodeRequests() would need to be exposed anyway, so that
DecodeStream knows how many output buffers it may need to buffer.

   //
   // Implementations guarantee that the callback will not be called from within
   // this method.
@@ -93,6 +95,9 @@ class MEDIA_EXPORT VideoDecoder {
   // use a fixed set of VideoFrames for decoding.
   virtual bool CanReadWithoutStalling() const;
 
+  // Returns how many frames that may be decoded in parallel.
+  virtual int GetMaxDecodeRequests() const;
+
  private:
   DISALLOW_COPY_AND_ASSIGN(VideoDecoder);
 };