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.