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/shared/background_hang_monitor/lib.rs

@@ -10,7 +10,6 @@ use std::time::Duration;
 use std::{fmt, mem};
 
 use base::id::PipelineId;
-use ipc_channel::ipc::IpcSender;
 use serde::{Deserialize, Serialize};
 
 #[derive(Clone, Copy, Debug, Deserialize, Serialize)]
@@ -170,7 +169,7 @@ pub trait BackgroundHangMonitorRegister: BackgroundHangMonitorClone + Send {
         component: MonitoredComponentId,
         transient_hang_timeout: Duration,
         permanent_hang_timeout: Duration,
-        exit_signal: Option<Box<dyn BackgroundHangMonitorExitSignal>>,
+        exit_signal: Box<dyn BackgroundHangMonitorExitSignal>,
     ) -> Box<dyn BackgroundHangMonitor>;
 }
 
@@ -208,6 +207,6 @@ pub trait BackgroundHangMonitorExitSignal: Send {
 pub enum BackgroundHangMonitorControlMsg {
     /// Toggle the sampler, with a given sampling rate and max total sampling duration.
     ToggleSampler(Duration, Duration),
-    /// Exit, and propagate the signal to monitored components.
-    Exit(IpcSender<()>),
+    /// Propagate exit signal to monitored components, and shutdown when they have.
+    Exit,
 }