Looking a little further, it appears to be a ProtocolBuffer file, but would a bit of documentation hurt?
https://chromium.googlesource.com/chromium/src.git/+/5d90890...
So the proto is a red herring, it's basically just a container for a blob in some other format.
https://chromium-review.googlesource.com/c/chromium/src/+/80... contains a lot of related files, including https://chromium-review.googlesource.com/c/chromium/src/+/80... which explains the format.
I'm surprised that there isn't a plain text version of the list checked in somewhere, but maybe I just missed it.
