KeyedService is 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. 

This component has the following structure:

- core/: shared code that does not depend on src/content/ or src/ios/
- content/: Code based on the content layer.
- ios/: Code based on src/ios.
