Like Update() but makes the gauge control run in indeterminate mode.
In indeterminate mode the remaining and the estimated time labels (if
- present) are set to to "Unknown" or to @a newmsg (if it's non-empty).
+ present) are set to "Unknown" or to @a newmsg (if it's non-empty).
Each call to this function moves the progress bar a bit to indicate
that some progress was done.