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/gfx/paint_thread.rs

@@ -435,7 +435,7 @@ impl<C> PaintThread<C> where C: PaintListener + Send + 'static {
             }
 
             debug!("paint_thread: shutdown_chan send");
-            shutdown_chan.send(()).unwrap();
+            let _ = shutdown_chan.send(());
         }, Some(id), panic_chan);
     }