background_hang_monitor: ensure workers run until monitored components do (#38322)

Shut-down of the background hang monitor(bhm) is currently  problematic:

- it does not always run until the monitored script-thread does(see
"BackgroundHangMonitor has gone away" mentioned in
https://github.com/servo/servo/issues/34158).
- it shuts-down before the constellation(good, so actually
https://github.com/servo/servo/issues/24850 was "fixed" but in a way
that introduced a new problem), but using a mechanism that allows it to
shutdown before script(the problem above).
- there are various mechanism(see the doc comments removed by this PR)
in place which are meant to ensure a clean shutdown despite the above
problems; those are complicated, and become unnecessary once those
problems are fixed.

All of the above is fixed by the changes in this PR, which ensure the
bhm does not shut-down before script, and also maintains the invariant
that it must shut-down before the constellation(in single-process mode)
or before the main thread(in multi-process mode), but using a mechanism
which allows it to keep running until script shuts-down.

An unnecessary option around the exit signal is also removed.

As a positive side-effect, it also ensures that any script-thread is
shut-down before the constellation(because for the bhm worker to exit,
the monitored script must have exited first), so this should also fix a
host of other problems noted in
https://github.com/servo/servo/issues/30849, but each should be
confirmed independently(and various other improvements seem possible in
their specific contexts, such as joining on script threads, and removing
the `ScriptThreadMessage::ExitScriptThread`).

Fixes: https://github.com/servo/servo/issues/24850 and part of
https://github.com/servo/servo/issues/34158

Testing: Unit tests in `component/background_hang_monitor/tests`. Also
manually tested loading "about-blank" in single- and multi-process mode.

---------

Signed-off-by: gterzian <2792687+gterzian@users.noreply.github.com>

components/constellation/pipeline.rs

@@ -5,6 +5,7 @@
 use std::collections::HashSet;
 use std::rc::Rc;
 use std::sync::Arc;
+use std::thread::JoinHandle;
 
 use background_hang_monitor::HangMonitorRegister;
 use background_hang_monitor_api::{
@@ -558,7 +559,7 @@ impl UnprivilegedPipelineContent {
 
     pub fn register_with_background_hang_monitor(
         &mut self,
-    ) -> Box<dyn BackgroundHangMonitorRegister> {
+    ) -> (Box<dyn BackgroundHangMonitorRegister>, JoinHandle<()>) {
         HangMonitorRegister::init(
             self.background_hang_monitor_to_constellation_chan.clone(),
             self.bhm_control_port.take().expect("no sampling profiler?"),