refactor: add SerialChooserController::web_contents_

Add a `base::WeakPtr<WebContents>` field to SerialChooserController and
stop subclassing from WebContentsObserver. This follows the Observer docs:

> don't create a `WebContentsObserver` just to be able to check
> for a null `WebContentsObserver::web_contents()`.
> Use a `base::WeakPtr<WebContents>` instead.