RouteObserverBehavior is now a ChromiumOS-specific utility which has moved from router.js into route_observer_behavior.js, so we need to migrate into using RouteObserverMixin instead. Chromium change: https://chromium.googlesource.com/chromium/src/+/775f9f5d2ca32cf0ab5545045301f8c5b9e6b310 commit 775f9f5d2ca32cf0ab5545045301f8c5b9e6b310 Author: dpapad <dpapad@chromium.org> Date: Mon Oct 11 08:40:24 2021 +0000 Settings: Move RouteObserverBehavior to OS Settings, only used there. Browser Settings no longer uses RouteObserverBehavior, as it has been migrated to RouteObserverMixin. Moving it to OS Settings is a preparatory step for migrating the shared router.js file to TypeScript. Bug: 1234307