When exiting, monitored component should unregister itself

[gterzian]

A component currently registered with the background-hang-monitor will start "hanging" after exiting, because it doesn't unregister itself, nor does it go into "waiting" mode, instead, it handles the last "exit" task, and quits. From the monitor's perspective, the component will later start hanging on that last "exit" task.

This will require:

1. Adding a Unregister message

   servo/components/background_hang_monitor/background_hang_monitor.rs

   Line 72 in c9229f3

   pub enum MonitoredComponentMsg {

2. Adding a unregister method, either for this or that trait(I would say rather BackgroundHangMonitor), as well as corresponding implementations.
3. When Layout exits, unregister it with the monitor.
4. When Script exits, do the same thing.
5. Add a test case, for example by asserting that no "hang alerts" are received from an unregistered component, which has just done a notify_activity, followed by unregistering itself, followed by either sleeping until the timeout is reached, or quitting.

[CYBAI]

@csmoe is working on it at #22474 so labeled as C-assigned.