Debugging UI

The login and lock screen can run with a developer overlay that make common operations such as setting up a device with 10+ users trivial. Pass the flag --show-login-dev-overlay when running chrome and the UI will automatically appear.

./out/Release/chrome --show-login-dev-overlay

The overlay will use fake data where necessary to show the relevant UI; buttons are mostly functional but may break since fake data may be sent to chrome.