Similar to  crbug.com/576817 , but for chrome.mdns.

Note that chrome.mdns.forceDiscovery is not the same as a getter - forceDiscovery will not result in onServiceList event being fired if the force-discovered service list is the same as before.

The new API will also help with crbug.com/647004 since that means we will no longer have to rely on the hack of remove / re-add listener to get the current list.