session_restore.startup_timeline is recorded once while the browser starts and cannot be
recorded again, so a test reading it depends on no other test in its manifest having called
Services.fog.testResetFOG(): browser-chrome gives one browser to a whole manifest.
browser_bfcache_telemetry.js legitimately resets, which is why this assertion, parked in
browser_reopen_all_windows.js by bug 1972836, read null and left that test permanently
intermittent. Asking the neighbours not to reset would keep the dependency working but leave
it invisible from the tests that have to respect it.
The FOG testing docs recommend testResetFOG without mentioning any of this, so they now say
that a test reading a startup metric needs a manifest of its own, and point at this one.
Differential Revision: https://phabricator.services.mozilla.com/D321930
Florian Quèze
c40ba6af47