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

services/media/framework/models/active_multistream_source.h

Index: services/media/framework/models/active_multistream_source.h
diff --git a/services/media/framework/models/active_multistream_source.h b/services/media/framework/models/active_multistream_source.h
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+++ b/services/media/framework/models/active_multistream_source.h
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+// 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_ACTIVE_MULTISTREAM_SOURCE_H_
+#define MOJO_MEDIA_MODELS_ACTIVE_MULTISTREAM_SOURCE_H_
+
+#include "services/media/framework/models/part.h"
+#include "services/media/framework/packet.h"
+
+namespace mojo {
+namespace media {
+
+// Asynchronous source of packets for multiple streams.
+class ActiveMultistreamSource : public Part {
+ public:
+  using SupplyCallback =
+      std::function<void(size_t output_index, PacketPtr packet)>;
+
+  ~ActiveMultistreamSource() override {}
+
+  // TODO(dalesat): Support dynamic output creation.
+
+  // Returns the number of streams the source produces.
+  virtual size_t stream_count() const = 0;
+
+  // Sets the callback that supplies a packet asynchronously.
+  virtual void SetSupplyCallback(const SupplyCallback& supply_callback) = 0;
+
+  // Requests a packet from the source to be supplied asynchronously via
+  // the supply callback.
+  virtual void RequestPacket() = 0;
+};
+
+}  // namespace media
+}  // namespace mojo
+
+#endif  // MOJO_MEDIA_MODELS_ACTIVE_MULTISTREAM_SOURCE_H_