www/chromium: Sync sndio backend with OpenBSD and unbreak build after r508722

../../media/audio/openbsd/audio_manager_openbsd.cc:158:14: error: allocating an object of abstract class type 'media::SndioAudioOutputStream'

return new SndioAudioOutputStream(params, this);
           ^

../../media/audio/audio_io.h:117:16: note: unimplemented pure virtual method 'Flush' in 'SndioAudioOutputStream'

virtual void Flush() = 0;

PR: 239802
Submitted by: tobik
Reported by: kib
Tested by: kib
Approved by: chromium (cpm)
MFH: 2019Q3 (together with r508722)