Signin is in the process of becoming a layered component (https://sites.google.com/a/chromium.org/dev/developers/design-documents/layered-components-design) to enable it to be shared cleanly on iOS. When this process is complete, this component will have the following structure: - core/: shared code that does not depend on src/content/ or src/ios/ - content/: Driver for the shared code based on the content layer. - ios/: Driver for the shared code based on src/ios.