This is the same bug at Ubuntu; it has more analysis:
https://bugs.launchpad.net/ubuntu/+source/pepperflashplugin-nonfree/+bug/1632870
In short:
* Google Chrome 54+ does not bundle libpepflashplayer.so.
* pepperflashplugin-nonfree will have to get it from somewhere else;
either adobe.com directly, or Google's "redirector.gvtl.com" thing.
It looks like Ubuntu don't care about pepperflashplugin-nonfree,
because they recommend adobe-flashplugin from canonical-partners.
https://wiki.ubuntu.com/Chromium/Getting-Flash
Debian has no equivalent.
