Bug 910210 - Overlay.update() should accept an optional parameter to override internal counter [r=arcturus]

When testing streaming import for massive vcard files, it is convenient to update the progress var in a set interval, to avoid updating it (and touching DOM) hundreds of times per second. It would be good to have an optional parameter to `update` in order to set the progress value, instead of calling the method every time the `counter` value is incremented by one.