An early version of the proto I found is: https://chromium.googlesource.com/chromium/src.git/+/5d9089080ea5fb0e348bb28f7a396d1181476f0f/chrome/browser/media/me

An early version of the proto I found is: https://chromium.googlesource.com/chromium/src.git/+/5d9089080ea5fb0e348bb28f7a396d1181476f0f/chrome/browser/media/media_engagement_preload.proto 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/+/803994 https://chromium-review.googlesource.com/c/chromium/src/+/80... contains a lot of related files, including https://chromium-review.googlesource.com/c/chromium/src/+/803994/9/tools/media_engagement_preload/make_dafsa.py 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.