Chrome resolved no default keychain for its own process, so before it could store its cookie key it put up a blocking "Keychain Not Found" panel with Cancel and Reset To Defaults, leaving the browser albedo drives stuck behind a dialog nobody asked for. A profile albedo owns has no business in the login keychain, and --use-mock-keychain keeps it out, on darwin only, so the automation browser cannot raise the panel at all. The owned argv moves into a module-level _command(), so a test reads the flags without starting a browser. browser_test.py asserts the darwin flag next to the rest of the owned options (profile, loopback debugging, headless, sandbox, trailing args, about:blank).