Creates a new offscreen document for the extension.
|parameters|: The parameters describing the offscreen document to create.
|callback|: Invoked when the offscreen document is created and has
completed its initial page load.
Determines whether the extension has an active document.
TODO(https://crbug.com/1339382): This probably isn't something we want to
ship in its current form (hence the nodoc). Instead of this, we should
integrate offscreen documents into a service worker-compatible getViews()
alternative. But this is pretty useful in testing environments.
|callback|: Invoked with the result of whether the extension has an
active offscreen document.
