There's a small clue at https://blog.chromium.org/2016/10/making-chrome-on-windows-faster-with-pgo.html https://blog.chromium.org/2016/10/making-chrome-on-windo

There's a small clue at https://blog.chromium.org/2016/10/making-chrome-on-windows-faster-with-pgo.html https://blog.chromium.org/2016/10/making-chrome-on-windows-f... : > To gather this data, the nightly build process now produces a special version of Chrome that tracks how often functions are used. It doesn't say how Chrome is exercised (what pages are loaded, what user actions are simulated) during the nightly build. It seems like it must be some kind of fixed test data. But that at least eliminates the possibility that they're gathering the profiling data from production builds used by real users, so the privacy concerns shouldn't be an issue. (Also note that info is from 2016, so it could have changed.)