Currently we have some issues with WebUI in extensions, particularly with the Bluettoth APIs.

Specifically:
* We use 'extension_id' throughout BluetoothEventRouter. Sometimes this is actually a 'listener id' and sometimes it is only an extension id (which may be problematic for WebUI. This should be audited and tested.
* We do not test support for multiple extensions or WebUI.
* We do not explicitly detect when a WebUI host becomes detached, we rely on all listeners being removed ( issue 703698 ) which is imperfect.
* The relationship between BluetoothAPI and BluetoothEventRouter is awkward; BluetoothAPI is an EventRouter::Observer and forwards events to BluetoothEventRouter which seems awkward. Also the lifetime of the event router and its reference to device::BluetoothAdapter is unclear.