Make VideoReceiveStream not inherit from I420FrameCallback.

There's no need for it as the current implementation only exists as a middle layer between the decoder and the eventual callback.

R=mflodman@webrtc.org, pbos@webrtc.org

Review URL: https://codereview.webrtc.org/2039053002 .

Cr-Commit-Position: refs/heads/master@{#13101}

webrtc/video/video_receive_stream.h

@@ -38,7 +38,6 @@ class VieRemb;
 namespace internal {
 
 class VideoReceiveStream : public webrtc::VideoReceiveStream,
-                           public I420FrameCallback,
                            public rtc::VideoSinkInterface<VideoFrame>,
                            public EncodedImageCallback,
                            public NackSender,
@@ -65,9 +64,6 @@ class VideoReceiveStream : public webrtc::VideoReceiveStream,
 
   webrtc::VideoReceiveStream::Stats GetStats() const override;
 
-  // Overrides I420FrameCallback.
-  void FrameCallback(VideoFrame* video_frame) override;
-
   // Overrides rtc::VideoSinkInterface<VideoFrame>.
   void OnFrame(const VideoFrame& video_frame) override;