Asynchronously polls this to see if media has been inserted or removed.
When the operation is finished, callback will be called. You can then call poll_for_media.end
to obtain the result of the operation.
| this |
a Drive. |
| cancellable |
optional Cancellable object, null to ignore. |
| callback |
a TaskReadyCallback, or null. |
| user_data |
user data to pass to |