Video playback broken on qt5-qtwebengine.

Under some circumstances, videos do not playback on qtwebengine-based browsers.

Error logs:

[1:20:0324/202035.328025:ERROR:render_media_log.cc(27)] MediaEvent: MEDIA_ERROR_LOG_ENTRY {"error":"FFmpegDemuxer: no supported streams"}
[1:1:0324/202035.329248:ERROR:render_media_log.cc(27)] MediaEvent: PIPELINE_ERROR DEMUXER_ERROR_NO_SUPPORTED_STREAMS

The issue can be reproduced by trying to play videos on Twitter using a qtwebengine based browser (e.g. Falkon).

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