Use RTP timestamp when checking for frame duplication.

Value of render timestamp can be the same for consecutive frames (e.g.
when old frames got decoded and need to be rendered immediately). It
should not be used for frame duplication checking.

Bug: b/122636276
Change-Id: Ie00bdd3fa50a2eacd48cba228fa3c54a6b206864
Reviewed-on: https://webrtc-review.googlesource.com/c/116790
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Commit-Queue: Sergey Silkin <ssilkin@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26210}

video/video_stream_decoder_impl.cc

@@ -276,6 +276,7 @@ void VideoStreamDecoderImpl::Decoded(VideoFrame& decoded_image,
             .set_video_frame_buffer(decoded_image.video_frame_buffer())
             .set_rotation(decoded_image.rotation())
             .set_timestamp_us(frame_timestamps->render_time_us)
+            .set_timestamp_rtp(decoded_image.timestamp())
             .set_id(decoded_image.id())
             .build(),
         casted_decode_time_ms, casted_qp);