Since prefs::mojom::PreferencesManager and prefs::mojom::PreferencesObserver form a 1:1 pairing, with bi-directional communication, we wish to enforce that binding has occurred before other api calls are made.

Currently this is not possible with creating a PreferencesFactory which would accept PreferencesObserver and return a created PreferencesManager.

However requiring a third mojom interface for all bi-directional communication is too much of a requirement for each client. Especially as each future process accessing preferences would need to have this overhead.

It is desired that service_manager::Connector provide an api to enforce the bi-directional binding at connection time. This will be brought up on chromium-mojo.

Once that is available we need to update the preferences code to use this new method. At that time we can stop forcibly checking the binding state in each method.

More details in code review: https://codereview.chromium.org/2474653003/