Allow signing into default profile and enabling extensions (#29) * Allow signing into default profile and enabling extensions - Add `signIn` argument to `startWithDebugPort` API. - Add tests. Related: https://github.com/dart-lang/webdev/issues/1490 * Addressed CR comments, fixed test failure * Addressed more CR comments * Addressed CR comments, fixed test flake * Create tem dir for eacch test, better flake fix * Make closing chrome on each test explicit