This adds `count` out parameter for recursive file remove calls to report the number of the removed entries.

  Having a lot of files removed by `::Remove(recursive=true)` has been a problem, as a slow disk can cause a hang with such call. A counter feature will help us knowing the situation better via telemetry.

  The use of out parameter here is to mark it optional and prevent unwanted changes in existing callers (because a return value can't be optional).

Differential Revision: https://phabricator.services.mozilla.com/D156940