We recently had some issues where opt-in to Sync Transport wouldn't persist across sessions, and while we ID'd a fix related to hashing, it would be best to have a more robust persistence test.

It's not clear if this is possible given the constraints of running tests, but one starting point would be imitating the StartupBrowserCreator tests:

https://cs.chromium.org/chromium/src/chrome/browser/ui/startup/startup_browser_creator_browsertest.cc?l=142&rcl=fb87be9f5c8801ba7ebb5478ad7f3dd447a33f3c