Creates a transparent interface between the chrome- and content contexts.
Calls to this object will be proxied via the message manager to a content frame script, and responses are returend as promises.
The argument sequence is serialised and passed as an array, unless it consists of a single object type that isn’t null, in which case it’s passed literally. The latter specialisation is temporary to achieve backwards compatibility with listener.js.
sendAsyncFn (function) – Callback for sending async messages.
browserFn (function) – Closure that returns the current browsing context.