Here's a more complete breakdown of the device code and what Monorail components I think we should have:

* Bluetooth
  - Cross-platform library (//device/bluetooth): IO>Bluetooth
  - Web Bluetooth: Blink>Bluetooth
  - Chrome Apps API: Platform>Apps>API>Bluetooth (existing issues are likely
    scattered between Platform>Apps>API and Platform>Extensions>API)

* HID
  - Cross-platform library (//device/hid): IO>HID (existing issues are filed
    against the Chrome Apps API but anything that is not specific to that API
    should be moved)
  - Web Authentication API (FIDO/U2F): Blink>WebAuthentication
  - Chrome Apps API: Platform>Apps>API>HID (existing issues are likely scattered
    between Platform>Apps>API and Platform>Extensions>API)

* Serial
  - There is only one consumer of the cross-platform library (//device/serial) so
    it doesn't get its own component.
  - Chrome Apps API: Platform>Apps>API>Serial (existing issues are likely scattered
    between Platform>Apps>API and Platform>Extensions>API)

* USB
  - Cross-platform library (//device/usb): IO>USB
  - WebUSB API: Blink>USB
  - Chrome Apps API: Platform>Apps>API>USB (existing issues are likely scattered
    between Platform>Apps>API and Platform>Extensions>API)

The components that don't exist in this plan are:
* IO>HID
* Platform>Apps>API>Bluetooth
* Platform>Apps>API>HID
* Platform>Apps>API>Serial
* Platform>Apps>API>USB