Started here: https://build.chromium.org/p/chromium.fyi/builders/CrWinClang64%20tester/builds/3239

FirstRunMasterPrefsWithTrackedPreferencesInstance/FirstRunMasterPrefsWithTrackedPreferences.TrackedPreferencesSurviveFirstRun/0 (run #1):
[ RUN      ] FirstRunMasterPrefsWithTrackedPreferencesInstance/FirstRunMasterPrefsWithTrackedPreferences.TrackedPreferencesSurviveFirstRun/0
[1560:1536:0516/194757:WARNING:chrome_browser_main_win.cc(420)] Command line too long for RegisterApplicationRestart
..\..\chromerowserirst_runirst_run_browsertest.cc(288): error: Value of: user_prefs->GetString(prefs::kHomePage)
  Actual: ""
Expected: "example.com"
..\..\chromerowserirst_runirst_run_browsertest.cc(289): error: Value of: user_prefs->GetBoolean(prefs::kHomePageIsNewTabPage)
  Actual: true
Expected: false
[  FAILED  ] FirstRunMasterPrefsWithTrackedPreferencesInstance/FirstRunMasterPrefsWithTrackedPreferences.TrackedPreferencesSurviveFirstRun/0, where GetParam() = "no_enforcement" (715 ms)

(looks like something is interpreting backslashes in path names incorrectly :-/)

That bot uses clang to build, but that's irrelevant. What's triggering the failure is that the bot does official builds. The "real" official builds will show this soon too.

This was likely triggered by https://codereview.chromium.org/1955773002 which added channel-specific code (which we never want to do btw: If something's ready to ship, we turn it on, and if it turns out to not be ready after all, we turn it off. We rely on the usual dev->beta->stable promotion. Explicit channel checks are an antipattern</aside>) and a field trial. This is probably doing the wrong thing in official builds somehow and might need a fix similar to https://codereview.chromium.org/1984583002