Motown: Add ActiveMultistreamSource model in preparation for the ffmpeg demux with async I/O

services/media/framework/models/multistream_source.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef MOJO_MEDIA_MODELS_MULTISTREAM_SOURCE_H_
 #define MOJO_MEDIA_MODELS_MULTISTREAM_SOURCE_H_
 
 #include "services/media/framework/models/part.h"
 #include "services/media/framework/packet.h"
 
 namespace mojo {
 namespace media {
 
-// Synchronous source of packets for multiple streams. This is currently used
-// by Demux, though it would be better if Demux were asynchronous.
+// Synchronous source of packets for multiple streams.
 class MultistreamSource : public Part {
  public:
   ~MultistreamSource() override {}
 
   // Returns the number of streams the source produces.
   virtual size_t stream_count() const = 0;
 
   // Gets a packet for the stream indicated via stream_index_out. This call
   // should always produce a packet until end-of-stream. The caller is
   // responsible for releasing the packet.
   virtual PacketPtr PullPacket(size_t* stream_index_out) = 0;
 };
 
 }  // namespace media
 }  // namespace mojo
 
 #endif  // MOJO_MEDIA_MODELS_MULTISTREAM_SOURCE_H_