Make VideoReceiveStream not inherit from I420FrameCallback.

webrtc/video_receive_stream.h

 /*
  *  Copyright (c) 2013 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 150 matching lines @@
     std::string sync_group;
 
     // Called for each incoming video frame, i.e. in encoded state. E.g. used
     // when
     // saving the stream to a file. 'nullptr' disables the callback.
     EncodedFrameObserver* pre_decode_callback = nullptr;
 
     // Called for each decoded frame. E.g. used when adding effects to the
     // decoded
     // stream. 'nullptr' disables the callback.
+    // TODO(tommi): This seems to be only used by a test or two.  Consider
+    // removing it (and use an appropriate alternative in the tests) as well
+    // as the associated code in VideoStreamDecoder.
     I420FrameCallback* pre_render_callback = nullptr;
 
     // Target delay in milliseconds. A positive value indicates this stream is
     // used for streaming instead of a real-time call.
     int target_delay_ms = 0;
   };
 
   // Starts stream activity.
   // When a stream is active, it can receive, process and deliver packets.
   virtual void Start() = 0;
   // Stops stream activity.
   // When a stream is stopped, it can't receive, process or deliver packets.
   virtual void Stop() = 0;
 
   // TODO(pbos): Add info on currently-received codec to Stats.
   virtual Stats GetStats() const = 0;
 
  protected:
   virtual ~VideoReceiveStream() {}
 };
 
 }  // namespace webrtc
 
 #endif  // WEBRTC_VIDEO_RECEIVE_STREAM_H_