A helpful note from an external dev of a Chromium fork:

"[...]AFAICT the flakiness I observed is caused by the updating sync instance
entering a CONFIGURING state, which will cause
ProfileSyncService::GetActiveDataTypes() to return an empty model type
set, and this in turn will ProgressMarkersMatch() (in the
QuiesceStatusChangeChecker file) to conclude that the engines match state,
and thus that the exit condition have been reached.

I am unsure whether the empty model type set should trigger a false return
 from the matcher, or if QuiesceStatusChangeChecker should check on the
ProfileSyncService::ConfigurationDone status; at present I have gone with
the latter option."

We should fix this.