The Java class ChromeSigninController [1] uses preference "google.services.username". However, the definition of the preference [2] states that it is deprecated, and only appears to be used in preference migration (from the C++ code).

I noticed this when updating the Clear Browsing Data dialog for Android [3], as it displays different text depending on whether the user is signed in or not. (I'm customizing the messages, so they need to be set from C++ code.)

cc'ing the original authors of [1] and [2] in case they have any insights.

[1] https://cs.chromium.org/chromium/src/components/signin/core/browser/android/java/src/org/chromium/components/signin/ChromeSigninController.java?l=18
[2] https://cs.chromium.org/chromium/src/components/signin/core/browser/signin_pref_names.cc?l=72
[3] https://cs.chromium.org/chromium/src/chrome/android/java/src/org/chromium/chrome/browser/preferences/privacy/ClearBrowsingDataPreferencesBasic.java?l=42