The current debian build, however, doesn't copy these files into /usr/lib/chromium, hence the error.
Adding this line to debian/chromium.install fixes the build: out/Release/*.bin usr/lib/chromium(It seems most logical to me to add it right after the out/Release/*.pak line, but it shouldn't matter where it gets added).
I added it, rebuilt the package, and can verify that the resulting chromium package now works.
smime.p7s
Description: S/MIME Cryptographic Signature