Same problem here. Sorry for my English but I think the problem is that chromium-67 is looking for ffmpeg-4 and the version available in Debian unstable is ffmpeg-3.4.2, I don't know if it's possible compile chromium-67 against ffmpeg-3.4.2. If it's not possible compile chromium-67 against ffmpeg-4 maybe this version and newer versions of chromium should go to the experimental branch temporarily (where there is already ffmpeg-4) or we have to wait for the transition of ffmpeg in Debian in order to have newer versions in unstable ( https://release.debian.org/transitions/html/auto-ffmpeg.html ).
Regards.
