Now that the API to clear all advertisements is available (crbug.com/636142), we need to consider where/when to use in the event of a Magic Tether crash.

It's difficult to determine whether we crashed recently or if we've left advertisements dangling. ChromeSessionManager::StartUserSession [1] initializes Magic Tether and is called after a crash (on real devices) so it's a decent first heuristic.

1) https://cs.chromium.org/chromium/src/chrome/browser/chromeos/login/session/chrome_session_manager.cc?q=chrome_session+package:%5Echromium$&l=116