It should be ported to use equivalent observer methods in IdentityManager::Observer, instead of GaiaCookieManagerService::Observer

See the 'Migration Guide' for specifics, including example CLs:
https://docs.google.com/document/d/1bO7JsZE-xA8DSE18NeCZffbAnFZTo6vAtBCKsxaz4u8/edit#heading=h.y21mzvk0pm4c