A stage of shutdown that supports blocker registration.
Wait for all blockers to complete.
Method aOnReady will be called once all blockers have finished.
The callback always receives NS_OK.
The blocker registration capability. Most services may wish to
publish this capability to let services that depend on it register
blockers.
The state of all the blockers of the barrier.
See the documentation of nsIAsyncShutdownBlocker for the
format.