Auto merge of #11585 - asajeffrey:constellation-avoid-deadlock-during-pipeline-closure, r=larsbergstrom

Avoid deadlock when closing a pipeline.

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At the moment, the constellation blocks on a pipeline during closure. This PR makes pipeline closure asynchronous.

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #11546.
- [X] These changes do not require tests because testing for absence of deadlock is difficult.

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components/script/script_thread.rs

@@ -1407,6 +1407,7 @@ impl ScriptThread {
         if window.pipeline() == id {
             debug!("shutting down layout for root context {:?}", id);
             shut_down_layout(&context);
+            let _ = self.constellation_chan.send(ConstellationMsg::PipelineExited(id));
             return true
         }
 
@@ -1414,6 +1415,7 @@ impl ScriptThread {
         if let Some(ref mut child_context) = context.remove(id) {
             shut_down_layout(&child_context);
         }
+        let _ = self.constellation_chan.send(ConstellationMsg::PipelineExited(id));
         false
     }